From SimplyRecipes.com; make sure you carmelize the onions completely, to bring out their best flavor
6 large red or yellow onions or a combination, peeled and thinly sliced.
1/4 teaspoon of sugar
2 cloves garlic, minced
8 cups of beef or chicken stock, or a combination of the two (traditionally the soup is made with beef stock)
1/2 cup of dry vermouth or dry white wine
1 bay leaf
1/4 teaspoon of dry thyme
Salt and pepper
8 slices of toasted French bread
1 1/2 cups of grated Swiss Gruyere with a little grated Parmesan cheese
1 In a large saucepan, sauté the onions in the olive oil on medium high heat until well browned, but not burned, about 30-40 minutes (or longer). Add the sugar about 10 minutes into the process to help with the carmelization.
2. Add garlic and sauté for 1 minute. Add the stock, vermouth or wine, bay leaf, and thyme. Cover partially and simmer until the flavors are well blended, about 30 minutes. Season to taste with salt and pepper. Discard the bay leaf.
3 To serve, you can either use individual oven-proof soup bowls or one large casserole dish. Ladle the soup into the bowls or casserole dish. Cover with the toast and sprinkle with cheese. Put into the broiler for 10 minutes at 350 degrees F, or until the cheese bubbles and is slightly browned. Serve immediately.