Begin by melting unsalted butter and olive oil in a large stock pot over medium heat.
Add the sliced onions to the pot and stir gently until they become tender and translucent, about 10-15 minutes.
Pour in the beef broth, along with dry sherry, dried thyme, salt, and pepper. Stir to combine and let it simmer for about 30 minutes.
Preheat your oven's broiler.
Ladle the hot soup into oven-safe bowls, place thick slices of French bread on top, and sprinkle with provolone, diced Swiss, and grated Parmesan cheese.
Place the bowls under the preheated broiler for about 2-3 minutes.
Remove the bowls from the oven and allow them to cool slightly before serving.