Hearty French Onion Soup Casserole

French fries. French toast. French onion soup.

I mean, can you think of anything that starts with the word “French” that doesn’t end up being something awesome? Well, maybe the French Revolution if you were the king…

Though I wonder if he had draped everyone in beef stock and Gruyère if things wouldn’t have turned out so bad? Unclear.

Look, we’re not here to discuss opposing sides intersecting in conflict. We’re here to celebrate two congruent comfort foods melding together into one harmonious work of art that’s meant to be enjoyed via spoon. I know what you’re thinking. How does soup (a typically watery liquid) transform into a casserole (a sturdy, thick dish)?

Magic, of course!

Okay, not really. The actual answer is flour, which, when you think about it—does contain its own element of wizardry in some ways. Flour is the ultimate thickener that takes a soupy-ish substance (soupy-ish being the technical term) and turns it into something with a little backbone.

Think of a roux which starts with equal parts fat (typically butter) and flour. A liquid (most often, milk, cream, or broth) is slowly streamed in and things take a turn for the creamy. Now here’s the trick with this soup casserole as the smoothness of the base is not exactly achieved in this order.

The foundation of standard French onion soup is simply made with beef broth and wine. I’m also a strong advocate for tomato paste here since it gives everything a complex, acidic boost and permits the broth the tenacity it needs to stick to your tongue. Double also, dry red wine and nutty, fortified Sherry make a lovely flavor profile—but white wine works just as well.

Whoops, I got carried away with aromatics.

In this recipe, we start by making the caramelized onions, which are cooked in butter (the mothership, I mean—the fat). Once those have reached their maximum intensity, in goes the flour—our magic thickening agent. The usual suspects are up next—beef broth and wine—but since we added flour, what was once regular French onion soup is now a creamy, dreamy pot of wine-infused caramelized onions and stock.

This velvety creation (pretty darn perfect on its own, if I do say so myself) is then met with two other common casserole supporting acts: bread and cheese. Instead of crispy, golden-brown baguette slices exclusively being placed on top, they’re weaved throughout in order to soak up every bit of savory broth.

If you’ve ever enjoyed French onion soup in a restaurant, you know that part of its charm is the stupefying entrance it makes as it arrives under your nose still-bubbling in a fiery hot individual crock made just for you.

Don’t worry, we don’t skip the “aha” moment here just because this masterpiece is meant to be shared.

Handfuls of sharp provolone and earthy Gruyère grace the top of the casserole (laying ever-so-gently on that top layer of bread) and after a kiss from the broiler, the spirited depth of the dish comes to life with an ooey, gooey sizzle, and rich, umami-like aroma that someone should definitely have made into an air freshener by now.

A bright hint of lemony thyme is laced throughout the casserole (and sprinkled on at the end for freshness and color), and just like that—you’ve completely forgotten that I started this story off by talking about a war in France.

Bon appetit, my friends.

Chef’s Tips

• To help the onions to caramelize a bit faster and encourage quicker browning, you can add a pinch of sugar or a drizzle of honey.
• To make this soup entirely vegetarian, use vegetable broth (preferably homemade as it will have much more depth of flavor than store-bought) instead of beef broth.
• If you can’t find Gruyère, substitute with a high-quality Swiss cheese to achieve the same nutty, melty effect. Avoid water-heavy cheeses like fresh mozzarella as it will tangle and almost dissolve into the soup instead of becoming stringy.

Step by Step Instructions with Photos

Step 1 – Caramelize the Onions

Slice the onions and chop the thyme.

In a heavy, wide-bottomed saucepot, add the butter over medium-low heat. Once it begins to foam and sizzle, add the onions and garlic.

Cook, stirring occasionally until the onions begin to get very soft and release their juices, about 20 minutes.

Add 1 teaspoon of the salt, the pepper, bay leaves, and 1 tablespoon of the thyme and continue to cook, stirring occasionally, until the onions are fully caramelized and deep golden brown, about 40 more minutes.

Step 2 – Make the Crostini

While the onions are caramelizing, toast the baguette slices. Preheat the oven to 350° F.

Arrange the baguette slices in a single layer on a baking sheet and drizzle both sides with the olive oil. Bake, flipping once halfway through, until golden-brown and toasted, about 10-12 minutes.

Step 3 – Build the Base of the Broth

Discard the bay leaves and stir in the tomato paste and the flour. Turn the heat to medium and cook, stirring occasionally, for 2 minutes.

Deglaze the pan, scraping up any bits from the bottom, with the Worcestershire, sherry, wine, and beef broth.

Bring the soup to a boil and stir constantly until lightened thickened, about 2-3 minutes, and then add the remaining 1 teaspoon of salt.

Step 4 – Layer the Crostini, Soup, and Cheese

In a 9x13-inch baking dish, layer the half of the toasted baguette slices and spoon half of the onion soup over top.

Evenly top with the remaining baguette slices and cheese.

Spread the cheese evenly over the top to cover the bread.

Step 5 – Bake and Serve

Cover the dish with foil and bake for 30 minutes. Remove the foil and broil the casserole until the cheese is bubbly and lightly golden.

Divide among bowls and top with the remaining 1 tablespoon of thyme.

Bonjour, Monsieur Casserole.

When it’s a toss-up between soup that goes in a bowl or a casserole that slides out of the oven, why limit yourself to just one cozy culinary journey?

This French onion soup casserole is truly the best of both worlds as the most important elements (sweet caramelized onions, bread, and cheese) still find a way to mingle for this meal.

If you’re cooking for someone who doesn’t mind meat but prefers chicken over beef, the chicken stock will hold up just fine. The end result won’t be as dark or fantastically fatty, but the fistfuls of cheese make up for that.

Pop these other party pleasers in the oven next time you’re looking to whip up some comfort for a crowd:

• Greek Moussaka Casserole
• Bean and Sausage Casserole
• Green Bean Casserole