French onion soup (or soupe à l’oignon gratinée, if you’re French) is a classic example of winter comfort food. One plunge of a spoon through the melted cheesy crust to the richly flavorful broth below and you’ll be hooked.
Legend has it that King Louis (either the 14th or 15th, take your pick) invented this dish after returning to his hunting lodge one night to find nothing in the pantry but onions, butter, and champagne. In reality it was probably created back in Roman times by a much more humble cook facing a similar predicament. This, like all truly great cooking, is a peasant dish born from necessity.
I love this recipe because at its core it’s about extracting as much complex flavor as possible from one ordinary ingredient, and that’s the ethos of Building Flavor! Now bear with me as I geek out a bit – after a small science lesson you’ll know why each step matters, which means you can cook this without being chained to the recipe.
Caramelization
The point of an onion bulb is to store enough energy for the plant to make it through the winter, and it keeps this energy in the form of sugars (mainly fructose, but also glucose and sucrose). When you heat up colorless, odorless sugar molecules, they break apart and magically recombine into a multitude of darker aromatic compounds, which depending on the molecule can taste buttery, vinegary, fruity, nutty, or toasty. This process is called caramelization.
The small amount of protein in the onions and the butter also combines with the sugars via the Maillard (browning) reaction which produces an even greater variety of flavor compounds, including savory, meaty, earthy, floral, and vegetal notes.
Finally, onions are full of nasty sulfur compounds that make them pungent and dissuade animals from digging them up and eating them. But when these compounds are heated… you guessed it, they also react and recombine into a whole other set of interesting flavors.
Fond and Deglazing
As caramelization and browning happens, you’ll begin to get a layer of brown gunk forming on the bottom of the pan. This is called fond (French for “base”) and it contains all those wonderful flavor molecules.
Once some fond has formed but before it has a chance to burn to a crisp, you deglaze the pan, which simply means adding a small amount of liquid like water, wine, or broth to dissolve the fond and mix those flavor molecules back into the dish. If you allow the liquid to evaporate, more fond will start to form and you can repeat the process, building another layer of flavor each time.
Plan of Attack
So, knowing all that, here’s what we need to do:
- Heat the raw onions until their cell walls start to break down and they soften, releasing liquid
- Once most of the liquid is evaporated, you’ll be able to get the onions up above the boiling point of water, into the temperature range where caramelization happens (220-340°F), and the onions will start to turn brown
- Get some fond to form and then deglaze it, repeat several times to keep building flavor.
- At this point, you have richly caramelized onions (you could steal some to add a tasty garnish to a sandwich or burger, or as a base for amazing onion dip)
- To your base of onions, add more flavors in the form of booze like sherry, wine, or cognac, meaty umami flavors from beef or chicken broth, and aromatic thyme. Top it off with water, and cook long enough that the flavor of the herbs and onions is drawn out and saturates the liquid. Now you’ve got soup!
- When serving, float some toast on top and cover with cheese (Gruyère, or Comté to be properly French about it) that melts well under the broiler. The toast keeps the cheese from sinking into the soup long enough to create that bubbly crust, and then magically dissolves to add body to the soup.
Because you’re working with a lot of onions, you have to do the first two steps slowly and evenly. If you dumped four pounds of onions into a pot over high heat and didn’t stir them, the onions on the bottom would be burned while the ones on top would still be raw.
There are a couple ways you can deal with this:
- High heat and constant stirring (which can be a pain in the ass, plus too hot and the onions can get bitter)
- Very low heat and a covered pot (which takes a long time)
- Putting the pot in the oven (so the heat comes from all directions, not just the bottom)
- And if you’re in a hurry, you can also get a head start by using the microwave for step one.
Here’s a version that uses the pot-in-the-oven method. Aside from the prep and deglazing it’s largely hands-off cooking.
| French Onion Soup | Print |
- 4 lbs yellow onions (about 6-8 large)
- 3 tablespoons unsalted butter
- salt
- ½ cup dry sherry or dry white wine
- 4 cups chicken broth (preferably low salt)
- 2 cups beef broth
- 2 cups water, plus more as needed for deglazing
- 1 bay leaf
- several sprigs of fresh thyme
- ground black pepper
- 1 baguette, cut into ½" slices
- ½ lb gruyere cheese, grated
- Arrange your oven racks so that the large pot will fit and pre-heat the oven to 400 degrees. Rub the bottom and sides of the pot with butter, or coat with non-stick cooking spray. Add the 3 tbsp butter to the pot.
- Slice the root and stem end off of each onion, then cut it in half pole-to-pole. Remove the skin, and slice each half pole-to-pole into ¼" slices. Dump these into the pot as you go.
- Cover the pot and place it in the oven. Cook for 1 hour. Pull the pot out (remember, it's hot so use pot holders or dry dish towels when you touch the handles or lid) Stir the onions well to mix them up - they should be softened but not yet brown.
- Put the pot back in the oven but this time crack the lid a bit to let steam escape. Cook 1 more hour. Pull the pot out again, and give them another stir. If they are mushy and golden brown, move on to the next step. Otherwise put them back in for another 15-30 minutes.
- Place uncovered pot on the stove over medium high heat. (Don't forget those oven mitts!) Keep stirring the onions and cook until they are dark brown and water evaporates. When a layer of fond has formed on the bottom of the pot, add a splash of water and scrape up the brown gunk until it's dissolved. Allow the water to evaporate and more fond will form - do this deglazing again 2-3 more times.
- After the water has evaporated and you've got some fond again, deglaze one last time with the wine. Stir until that has evaporated, then add the 2 broths, 2 cups water, bay leaf, thyme, and a dash of salt. Scrape up any last bits of fond, increase the heat to high until it's almost boiling. Reduce the heat to low, put the lid on, and simmer for half an hour.
- While the soup is simmering, place the baguette slices on a baking sheet and place them in the 400 degree oven for 10-15 minutes until they are lightly golden brown.
- When soup is done, use tongs to pull out the thyme and bay leaf if you can find them, then add salt and pepper to taste. Ladle out the soup into your oven safe crocks, then cover each bowl with 1-2 baguette slices, and bury the whole thing with grated cheese. Place them under the broiler until the cheese is melted and bubbly, with patchy brown spots. Let cool for a few minutes before serving - these will be molten hot!
If you don’t have any oven-safe crocks, you can keep the cheese-covered bread slices on the baking sheet and toss that under the broiler instead. Then serve your soup in regular bowls and float a piece or two of cheezy bread on top.
If you have time and really want to boost the flavor, you can make home made beef stock like Julia Child did. But that’s a post for another day. You can also experiment with mixing different kinds of onions, or even onion relatives like shallots, leeks, or small amounts of garlic. Do you have a favorite variation? If so, let me know in the comments!
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