If you've got leftover caramelized onions, or you are about to make a recipe that uses them, double up. And while you're at it, grab that loaf of leftover bread because you are about to embark on a trip to France in one delicious vegetarian casserole based on the famous soupe à l'oignon gratinée.
Caramelized Onions Are a Leftover Rockstar
Caramelized onions are a result of that magical mixture of onions and time, cooked slow and on a low heat, so the onion's natural sugars come forward and the onions become jammy. This recipe includes making caramelized onions for the base of the casserole, but I do my best to have some on hand almost all the time. Caramelized onions also keep, refrigerated, for about three days in a covered container, and can even be frozen.
A Mini Guide to Caramelized Onions
Stock: The Foundation Flavor in a Vegetarian Casserole
To stock or not to stock? That is the question raised by so many chefs and cooks when it comes to French onion soup's base. Most onion soups are made with a beef stock, made from browned bones and plenty of vegetables and cooked ad infinitum. This recipe creates its own vegetable stock as you cook the ingredients. It takes less than half an hour and works as a wonderful, supportive base for the incredible caramelized onions. If you start the onions from scratch, assume it's about an hour of cooking time.
The Joys of Day-Old Bread
Second-day bread is a kitchen wonder. This recipe puts that loaf of bread to full use — creating a mini-grilled cheese sandwich — all right on top of the casserole to soak into the deliciousness below.
French Onion Soup Casserole
For the stew:
8 tablespoons (1 stick) unsalted butter, divided
7 large yellow onions (5 to 5 1/2 pounds total), halved and cut into 1/4-inch-thick slices
4 large shallots (about 8 ounces), peeled and cut into small dice
4 teaspoons kosher salt, divided
1 teaspoon freshly ground black pepper, divided
6 cloves garlic, minced
5 stalks celery (about 12 ounces), including leaves, cut into small dice
5 carrots (about 8 ounces), peeled and cut into small dice
6 cups cremini and/or button mushrooms (about 1 1/2 pounds), sliced thinly
1/4 cup all-purpose flour
1 cup dry Riesling wine
6 to 8 fresh sage leaves, minced, or 2 teaspoons dried sage
Leaves of 5 sprigs fresh thyme, finely chopped
1 (15-ounce) can white beans and their liquid (optional)
For the topping:
1 large baguette (about 1 pound), cut crosswise into 24 slices
12 ounces Gruyere, Comté, or Raclette cheese, shredded
4 tablespoons unsalted butter
Make the stew: Heat 4 tablespoons of the butter in a large, heavy-bottomed pot or Dutch oven over medium-high heat until melted and just beginning to foam.
Add the onions, shallots, 1 teaspoon of the salt, and 1 teaspoon of the pepper. Cook, stirring occasionally, until the onions are translucent and begin to brown on the edges, 5 to 7 minutes. Reduce the heat to low (or the lowest your stovetop will go) and cook, stirring occasionally and adjusting the heat as needed, until the onions are completely softened and caramel brown in color, about 30 minutes.
Add the garlic, celery, carrots, and mushrooms (add the mushrooms in batches if they do not all not fit into your pot at once, as they will cook down and reduce in volume), and stir to combine. Once you have added all the mushrooms, cover and cook 20 minutes more.
Arrange a rack in the middle of the oven and heat to 375°F. Coat a 9x13-inch baking dish with cooking spray and set aside.
When the vegetables are ready, transfer them with a slotted spoon to a large bowl and set aside. Pour the remaining vegetable cooking liquid into another bowl or a measuring cup with a spout; set aside.
Return the pot to high heat, add the remaining 4 tablespoons of butter, and cook until it is melted and begins to foam. Add the flour, whisk to combine, and cook until it is golden-blonde in color, about 2 minutes. Add the wine and whisk to combine.
Add the reserved vegetable cooking liquid and whisk to combine. Add the sage, thyme, and remaining 3 teaspoons salt and whisk to combine. Add the reserved cooked vegetables and beans and their liquid, if using, and stir to combine. Pour the mixture into the casserole dish and spread into an even layer; set aside.
Make the crouton topping: Arrange 12 of the bread slices on top of the casserole in a single layer, pushing them down gently into the liquid and vegetables. Sprinkle with about half of the cheese. Arrange the remaining 12 slices of bread over the cheese in a single layer, then sprinkle the remaining cheese on top. Melt the butter in the microwave, then drizzle over the casserole.
Bake until the cheese is melted and the casserole is bubbly, 20 to 22 minutes. Serve immediately.
- Make ahead: This recipe can be broken into parts and cooked over several days: Make the onions one day, cook it with the vegetables the next, and make the sauce and put the casserole together on the final day. Reheat the vegetable mixture before finishing the casserole.
- Slow-cooker vegetable preparation: You can also make this recipe a second way, as a make-ahead, if you are short on time. Prepare the onions as written, but then transfer them and all the vegetables to a slow cooker, stir, and set at low or medium-low and cook for 3 to 4 hours. Then scoop the vegetables out, make the roux (flour and butter mixture) on the stove, add the wine and vegetable cooking juices to the pan to make the sauce, and add the cooked vegetables as written.
- Storage: This dish is best eaten fresh, as the bread absorbs liquid as it sits and the casserole will get soggy. If you have any leftovers, they can be stored in an airtight container in the refrigerator for up to 3 days.