Melt the butter and olive oil in a large pot over medium heat.
Add the sliced onions and sprinkle with salt.
Cook the onions until they turn a deep golden brown (about 30-40 minutes).
Add the sugar and stir well to incorporate.
Pour in the white wine, scraping the bottom of the pot.
Add the beef broth, thyme, and bay leaf, then bring to a simmer.
Let the soup simmer for about 30 minutes, adjusting seasoning as necessary.
Preheat your broiler while you prepare the baguette slices.
Ladle the soup into oven-safe bowls, place a slice of toasted baguette on each, and top with cheese.
Broil the bowls for 3-5 minutes or until the cheese is bubbly and golden.