1. Heat the butter and oil together in a Dutch oven over medium heat. Add the onions, shallots and leek, and season with salt and pepper, to taste. Stir to combine and cover. Cook, without stirring until the onions are soft and beginning to brown, about 10 to 15 minutes.
2. Uncover, and continue to cook, stirring often, until the onions are a rich brown color and well caramelized, about 25 to 30 minutes. (If the onions begin to stick to the bottom of the pan or burn add about a 1 tablespoon water at a time.) Meanwhile, tie the reserved leek top with the thyme and rosemary using a piece of kitchen twine.
3. Sprinkle the flour over the onions and cook, stirring, to cook out the raw flour taste, about 2 to 3 minutes. Stir in the broth and 2 cups water, scraping any browned bits from the bottom of the pan; add the herb bundle. Reduce the heat and simmer until thickened, 20 to 25 minutes. Stir in the Worcestershire and remove from the heat. Discard the herb bundle, stir in the sherry and season, to taste, with salt and pepper.
4. Preheat the broiler. Divide the soup between 4 (16-ounce) oven proof bowls or ramekins and arrange them on a baking sheet. Top each bowl evenly with 2 baguette slices and top with the cheese. Put under the broiler until the cheese is melted and lightly browned, about 1 minute. Sprinkle with the chives and serve.
Serving Suggestion: Turn this rich soup into a full meal with a salad of baby mixed greens, sliced endive and beets with a lemon vinaigrette.
Copyright 2010 Television Food Network, G.P. All rights reserved