- 6 large yellow onions
- 3 tablespoons olive oil, plus extra for brushing
- One 14-ounce can chicken broth
- One 10.5-ounce can beef broth
- 2 tablespoons Marsala or red wine (optional)
- Four to six 1-inch-thick slices French bread
- 8 ounces Gruyere cheese, cut into thin slices
- Peel and thinly slice the onions by hand or in a food processor. Put in the slow cooker and toss with the olive oil. Cover and cook on LOW for 3 to 4 hours, stir, and continue cooking several hours, until the onions are well cooked and caramelized. Total cooking time will be 8 to 10 hours. When the onions are done, add both the broths and Marsala, if using; add no water or salt. Cover and cook on HIGH until hot, 15 to 30 minutes.
- Meanwhile, preheat the oven to 400°F. Put the bread slices on a baking sheet and brush with olive oil or leave plain. Bake until golden brown around the edges, about 10 minutes. Set aside.
- Ladle the soup into individual ovenproof soup bowls and place on a baking sheet to prevent the bowls from tipping over. Top each bowl full of soup with a toasted bread slice and cover with a slice of the Gruyere. Adjust the oven rack to comfortably fit the bowls under the broiler with at least 4 inches to spare. Turn on the broiler and slide the soups under the flame; broil until the cheese is melted and bubbly, 1 to 2 minutes. Serve immediately.