The key to this classic comforting soup, made with everyday ingredients, is taking the time to caramelize the onions until they are tender, sticky and sweet. The mushrooms are not traditional but add a welcome touch of umami. Making the base rich in flavor before adding beef broth, preferably homemade, or a high-quality store-bought version, will reward you for your efforts. Use low-sodium chicken broth or a half-beef and half-chicken mixture; consider adding a pinch of Worcestershire and a cube of beef stock, in a pinch. Hot, bubbling cheese crowns a golden crouton cut just thick enough for the bottom half to absorb the rich broth while the top layer turns golden and melts when toasted just before serving.
Onion and mushroom soup with cheese croutons
Makes 4-6 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 broth or half beef/half chicken broth
2 garlic cloves, minced
2 to 3 sprigs of fresh thyme or 3/4 teaspoon of dried thyme, Italian seasoning or Provencal herbs
Baguette, 8 to 10 slices, cut 1 inch thick
8 ounces cheese, such as Gruyere, Comté, or Fontina, grated or sliced
Peel the onions, halve them lengthwise and cut each half into thin slices. In a large, heavy-bottomed saucepan (6 quarts), heat the olive oil over medium heat. Add the onion slices and toss to coat them with the oil. Cook, stirring often, until onions have softened, 20 minutes (after 10 minutes, increase heat to high and cook, stirring often, 10 minutes). Add the mushroom slices, butter, 1/2 to 3/4 tsp salt (depending on the sodium content of the broth) and 1/2 tsp freshly ground black pepper. Cook over medium to medium-high heat, stirring often, 10 minutes. Sprinkle the onions with a teaspoon of sugar, if desired, to help with caramelization; cook, stirring often, maintaining the heat between medium-high and high, until the onions are golden brown, about 10 minutes more. Add balsamic vinegar or Worcestershire sauce, if using. Deglaze by adding the wine and scraping the bottom and sides of the pan. Let cook for a minute. Add broth, garlic and thyme. Bring to a simmer; reduce heat to low; cover and simmer for about 30 minutes, checking after a few minutes that the soup does not boil. Taste and adjust seasoning by adding more salt, pepper or garlic as needed.
While the soup cooks, cut the baguette into 1-inch-thick slices and brush both sides with a little olive oil; place the slices on a baking sheet. Bake at 450 degrees for five minutes or until lightly browned. Remove the baking sheet from the oven. Using tongs, flip the slices. 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 the soup in ovenproof bowls and garnish with toasted baguette slices; top with grated cheese and broil until cheese is bubbly.