Thinly slice the onions. In a large stockpot, over medium high heat, heat the oil and sauté the onions until golden brown, about 20 minutes. Deglaze the pan with port. Add the stock, salt, pepper, and bouquet garni and simmer for 30 to 40 minutes. Dry the slices of French bread in a warm oven. Place 1 slice in each of 6 oven-proof bowls. Pour the soup over the bread. Top each bowl with 1 slice of bread and sprinkle with 1/3 cup of cheese. Brown under the broiler.