Melt butter and one tablespoon of olive oil in a large pot (or Dutch oven) over medium-low heat.
Add onions and brown sugar. Stir well. Cover and sauté gently on medium-low heat for 15 minutes. Stir occasionally.
Remove the cover and increase heat to medium and cook uncovered for 15 minutes. Stir occasionally.
Add bay leaves and thyme and deglaze with dry sherry and marsala wine. Add beef broth and bring to a quick boil. Reduce heat and simmer for an additional 15 minutes.
Brush sliced baguette with olive oil and toast at 400º Fahrenheit until crispy. Remove and set aside.
Ladle the soup into the soup bowl, top with two slices of toasted baguette, and sprinkle with Gruyere cheese. Heat for 2-3 minutes under the broiler until the cheese is bubbling, melted, and lightly browned.