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 French onion soup.
Caramelized onions are a result of that magical mixture of onions and time, cooked slowly on 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 try to have some on hand all the time. Caramelized onions keep, refrigerated, for about three days in a covered container, and can even be frozen.
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 will take it’s about an hour of cooking time.
Second-day bread is a kitchen wonder. This recipe puts that loaf of bread to full use – creating a mini-grilled cheese sandwich – right on top of the casserole to soak into the deliciousness below.
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This dish is best eaten fresh, as the bread absorbs liquid as it sits and the casserole will get soggy. But if you have leftovers, store them in an airtight container in the refrigerator for up to three days.
French Onion Soup Casserole
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 diced
4 teaspoons kosher salt, divided
1 teaspoon freshly ground black pepper, divided
6 cloves garlic, minced
5 stalks celery (about 12 ounces), including leaves, diced
5 carrots (about 8 ounces), peeled and diced
6 cups 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 into 24 slices
12 ounces Gruyere, Comte or Raclette cheese, shredded
4 tablespoons unsalted butter
Heat 4 tablespoons 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 salt and 1 teaspoon 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 and cook, stirring occasionally and adjusting the heat as needed, until the onions are completely softened and caramel brown in color, about 30 minutes. (Can be made ahead to this point.)
Add the garlic, celery, carrots and mushrooms (add the mushrooms in batches if needed; they’ll cook down and reduce in volume), and stir to combine. Once you have added all the mushrooms, cover and cook 20 minutes.
Heat oven to 375 degrees. Coat a 13-by-9-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 butter, and cook until it is melted and begins to foam. Whisk in the flour and cook until it is golden-blonde, about 2 minutes. Whisk in the wine.
Whisk in the reserved vegetable cooking liquid. Add the sage, thyme and remaining 3 teaspoons salt. Add the reserved cooked vegetables and the beans and their liquid, if using, and stir to combine. Pour into the casserole dish and spread into an even layer; set aside.
Make the topping: Arrange 12 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, then sprinkle with the remaining cheese. Melt the remaining butter, then drizzle over the casserole.
Bake until the cheese is melted and the casserole is bubbly, 20 to 22 minutes. Serve immediately.
Slow-cooker: Prepare the onions as directed. Transfer them and all the vegetables to a slow cooker, stir, and set at low and cook for 3 to 4 hours. 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. Spread in the baking dish and make the topping as directed.
Yield: 8 servings.