I prefer to top mine with some Parmesan and Mozzarella.
4 Tbsp. unsalted butter
2 lbs. sweet onions (2 large) halved and sliced thin
Salt and Pepper
1 Tbsp. packed brown sugar
1 tsp. minced fresh thyme
3/4 cup dry red wine
2 cups beef broth
2 cups chicken broth
2-3 heaping Tbsp. flour
1/4 cup soy sauce
1 small baguette, cut into 1/2-inch slices
Shredded cheese of your choice (Swiss, Provolone, Mozzarella)
Add butter to stock pot and cook until melted. Add onions, 2 tsp. salt, 1 tsp. pepper, brown sugar and thyme. Cook onions over medium heat until onions are softened and deep golden brown (20-30 minutes.
Add stock and bring to a simmer. Simmer gently for 20-30 minutes. Mix flour and soy sauce to form a slurry. Whisk in to soup and whisk until smooth. Cook an additional 5 minutes.
Preheat oven to 400 degrees F. Arrange bread slices in single layer on baking sheet and bake until bread is golden at edges, about 10 minutes.
Turn oven to BROIL.
Divide toasted bread into soup bowls or crocks. Top with a sprinkling of parmesan cheese.
Ladle hot soup into bowl until full. Top with plenty of mozzarella (or cheese of your choice), and broil until cheese is melted and lightly browned.