Melt the butter together with the olive oil in a Dutch oven over medium heat, then add the white and red onions, leeks, garlic, bay leaves, thyme and salt and pepper to taste, and cook, covered, stirring occasionally, until all the onions are caramelized, about 30 minutes.
Add the sherry and cook until reduced by half, 10 to 15 minutes. Discard the bay leaves and thyme sprigs. Add the beef stock, bring to a simmer and cook 15 minutes. Taste and adjust the salt and pepper as desired.
Preheat the broiler. Add the baguette toasts to each soup bowl and ladle soup over the top (they will rise to the top). Top each toast with the cheese and broil until the cheese browns, about 30 seconds.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Sidewinders American Grill, Jackson Hole, WY