Easiest French Onion Soup
- Peel and thinly slice the onions by hand or in a food processor. Put in the slow cooker and toss with olive oil. Cover and cook for 9 to 10 hours on High.
- Add both the broths and Marsala, if using; add no water or salt. Cover and continue to cook on High until hot, 15 to 30 minutes.
- Meanwhile, preheat the oven to 400 degrees 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. 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.