French Onion Soup Secret Ingredient

In today’s post we’re revealing our French onion soup secret ingredient! Actually,  there’s more than one secret ingredient in this Unknown Chef exclusive recipe and we’re divulging them all.

When it comes to the best french onion soup recipe, I believe Hubs has this hands down! He’s an onion aficionado and honed this recipe for years. Hubs had to experiment and eat a lot of bowls of French onion soup to settle on his best recipe. Tough job, I know!

After keeping it to himself for so long, he’s finally ready to share it. If you love onion soup a much as we do, we hope you’ll try it and let us know what you think! We’re sharing all our tips for the best bowl of french onion soup:

French Onion Soup Secret Ingredient

#1 – Two Kinds of Onions

The best onion soup is made with not one variety of onion, but two! And lots of them!

You’ll need a profuse amount of onions to make this recipe and it does take a long time to cook them down. However, your patience will be rewarded. That’s because there’s a lot of flavour in caramelization and you can only achieve that with time.

The secret is to use red onions in combination with either Spanish or Vidalia.

French Onion Soup Secret Ingredient #1 is two kinds of onions: white and red

#2 – Port

If you are a teetotaler, all I can say is you may never experience the best onion soup ever. There are plenty of onion soup recipes advocating white wine over red wine and vice versa. But nothing compares to the robust flavour of Port.

When Hubs was studying for his chef’s certificate, he took two courses to learn about wine. Port was a clear winner for this recipe because it fits right in with the sweetness of the caramelized onion. Especially after a trip to Portugal, Taylor Fladgate Port has had a slight edge over other ports as a mainstay of the Unknown Chef’s french onion soup recipe. Choose a port that you love and you can’t go wrong.

#3 – Apple Cider

Using unfiltered apple cider amps up the bold rich flavour of this onion soup. Keep layering on that flavour!

UPDATE: please don’t mistake apple cider for apple cider vinegar. There is NO vinegar in this recipe.

#4 – Fontina Cheese

While gruyere is traditional in onion soup, a rich, creamy, nutty cheese like fontina will balance the robust flavour of this onion soup. More about Fontina cheese here.

#5 – Stock up

Hubs has made a vegetarian version of this with vegetable stock and white wine, but I don’t consider that the best ever french onion soup. On the other hand, a combo of beef and chicken stock really clicks. The secret is to stock up on stock: use homemade stocks, where possible, for best results. We always make homemade chicken stock from scratch. Admittedly, we have cheated with a powdered beef broth on occasion. Regardless of whether it’s home-made or not, don’t be caught asking yourself ‘where’s the beef’ (and chicken in this instance).

#6 – Skip the Bread

Growing up, I frequently ordered french onion soup in restaurants. However, it was always served it with ‘toasty’ bread and cheese floating on top. Of course, I loved the cheese; hated how the bread would get soggy. No matter how you time it, the bread doesn’t hold up well in the liquid.

So what’s the Unknown Chef substitute? Crack an egg on top, broil it and melt the cheese over that. Hubs used to stir the egg into the soup, like an egg drop soup, but now his preference is to leave the egg whole. Adjust the recipe the way you like it, or leave it out altogether!

While you might miss the crunch factor, the yolk from the egg adds extra creaminess (if you leave it whole). But if you really do want the crunch factor, add the toasted bread at the last possible second and eat immediately.

French Onion Soup Secret Ingredient Recipe

Follow this recipe and you’ll end up with a deep mahogany flavour bomb. For the two different types of onions select the 5 biggest ones; the bigger the better.

wooden bowls of red onions of varying sizes

When cutting the onions, be sure to form your fingers into a ‘claw’. When the knife butts up against your knuckles, you can’t accidentally cut your fingers.

hand holding knife slicing onion on cutting board

Once you crack an egg in, cook it then add the cheese, put it back into the oven until it’s golden and bubbly.

Bowl of French onion soup on wooden board with melted cheese on tp

Source: Photo by sheri silver on Unsplash

Keep the soup for up to 3 days in the fridge or freeze for up to 6 months.

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Onion Soup FAQs

What onions are best for onion soup?

You don’t have to pick just one type! For the best bowl of onion soup, we find that a combination of red onion and either Spanish or Vidalia provide outstanding flavour.

How Do I Thicken French Onion Soup?

The best way to thicken the soup is by cooking the onions a long time and reducing the liquid slowly. You’ll end up with a rich robust flavour that only time can develop.

Why Does My French Onion Soup Taste Bitter?

Onions caramelize to a sweet rich flavour when cooked down correctly. If you rush the cooking process, or raise the temperature too high, the sugars in the onion will burn and turn bitter.

French Onion Soup

Cook Time 1 hour 35 minutes


  • 5 large Spanish or Vidalia onions
  • 5 large red onions
  • 3 TBL butter
  • 1 cup port wine
  • 10 oz beef stock
  • 10 oz chicken stock
  • 10 oz apple cider* unfiltered
  • Bouquet garni: thyme sprigs bay leaf and parsley wrapped in a bundle of cheesecloth
  • Eggs one per serving
  • Kosher salt
  • Ground black pepper
  • 1 cup fontina or gruyere cheese grated


  • Cook to an even, deep golden brown - not black or burned!
  • Trim the ends from each onion, then slice in half from top to bottom. Remove peel and finely slice into half moons.
  • Set electric skillet to 300 degrees and add butter. Once butter is melted, add a layer of onions and sprinkle with a little salt. Repeat layering onions and salt until all onions are in the skillet. Put lid on. do not stir until onions have sweated down for 20 to 30 minutes.
  • After 30 min. remove lid and keep stirring occasionally until onions are dark mahagany and reduced to approximately 2 cups. This should take about 45 min. to 1 hour, stirring often near the end to avoid burning.
  • Add port wine to cover onions and continue stirring until wine is reduced to a syrup consistency. Add chicken and beef broth, apple cider and bouquet garni. Reduce heat and simmer 15 - 20 min., then remove herbs.
  • Taste and adjust with salt and pepper.
  • To serve, ladle hot soup into oven proof bowls. You can prepare the egg like an egg drop soup or leave it whole. For the former, crack an egg into a cup, lightly stir and pour onto top of soup in bowl. For the latter, drop in the whole egg.
  • Broil in oven or toaster oven until egg is cooked. Top with grated cheese, lightly broil again until cheese is melted and golden brown.


The Unknown Chef Tips:
  • Make soup one day ahead to allow to thicken in fridge
  • A large deep electric frying pan with lid is essential for this soup. Fill the pan with onions above the brim so lid just fits to make a thick soup.
  • Freezes well; store flat in zip lock freezer bags
  • Ensure soup is hot before adding egg to cook it through
  • Smoked cheese works well too as a substitute
  • Taylor Fladgate or Grahams LBV Port are good choices… serve a small glass with soup and enjoy!



4 thoughts on “French Onion Soup Secret Ingredient

    • It’s at the top of our list too. I forgot to mention that it freezes well so you can always have some on hand.

    • People were literally gagging…SO vinegar strong. Could’ve eat, what a waste of time AND MONEY! Followed recipe exactly…all afternoon!

      • Hi Kathy – sorry this turned out to be a waste but you actually didn’t follow the recipe. There is NO vinegar in this recipe, just apple cider. I will add a note so others don’t make the same mistake :).

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