The key to this classic comfort soup, made of everyday pantry ingredients, is taking the time to caramelize onions until they become tender, sticky and sweet. Mushrooms aren’t traditional but add a welcome burst of umami. Making the flavor-rich base before adding beef stock, preferably homemade, or a high-quality store-bought version, will reward you for your efforts. Use low-sodium chicken stock or a combo of half beef and half chicken; consider adding a dash of Worcestershire and a beef bouillon cube, in a pinch. Hot, bubbling cheese crowns a golden crouton cut just thick enough so that the bottom half soaks up the rich broth while the top layer gets golden and melted when broiled just before serving.
French Onion and Mushroom Soup with Cheesy Croutons
Makes 4 to 6 servings
5 large yellow or red onions (3 to 3 1/2 pounds)
3 tablespoons extra-virgin olive oil
16 ounces mushrooms, such as cremini, shiitake, morel, or King trumpet, trimmed and sliced (about ¼-inch thick)
2 tablespoons unsalted butter
Salt and freshly-ground black pepper
1 teaspoon balsamic vinegar or Worcestershire sauce (optional)
1/2 cup dry white wine, dry sherry, or vermouth
8 cups low-sodium beef stock or half beef/half chicken stock
2 cloves garlic, minced
2 to 3 sprigs fresh thyme or 3/4 teaspoon dried thyme, Italian seasoning or herbes de Provence
Baguette, 8 to 10 slices, cut 1-inch thick
8 ounces cheese, such as Gruyère, Comté, or Fontina, grated or sliced
Peel onions, cut in half lengthwise, and slice each half into thin slices. In a large (6-quart), heavy-bottomed pot, heat olive oil over medium heat. Add onion slices and stir to coat with oil. Cook, stirring often, until onions have softened, 20 minutes (after 10 minutes, increase heat to high and cook, stirring often, 10 minutes). Add mushroom slices, butter, 1/2 to 3/4 teaspoon salt (depending on sodium content of stock) and 1/2 teaspoon freshly-ground black pepper. Cook over medium to medium-high heat, stirring often, 10 minutes. Sprinkle onions with one teaspoon sugar, if desired, to help with caramelization; cook, stirring often, keeping heat between medium-high and high, until onions are a deep golden brown, about 10 more minutes. Add balsamic vinegar or Worcestershire sauce, if using. Deglaze by adding wine and scraping bottom and sides of pot. Let cook one minute. Add stock, garlic, and thyme. Bring to a low boil; reduce heat to low; cover, and let simmer for about 30 minutes, checking after a few minutes to make sure soup isn’t boiling. Taste and adjust seasoning, adding more salt, pepper, or garlic, as needed.
While soup is cooking, slice baguette into 1-inch thick slices and brush both sides with a little olive oil; place slices on a baking sheet. Bake at 450 degrees for five minutes or until lightly browned. Remove sheet pan from oven. Using tongs, turn slices over. Sprinkle with grated/sliced cheese and bake until cheese is bubbling and beginning to brown. Ladle soups into individual bowls and serve with croutons on top.
Alternatively, serve soup in ovenproof bowls, and top with toasted baguette slices; top with grated cheese and broil just until cheese is bubbly.