Slice 4 large onions thinly and melt 2 tablespoons of butter in a large pot or Dutch oven over medium heat. Add the sliced onions to the pot and cook slowly, stirring occasionally, until they turn a deep golden brown and become sweet and tender, which typically takes 30-40 minutes.

Pour in 1/2 cup of dry white wine or vermouth to deglaze the pot, scraping up any browned bits from the bottom, then add 6 cups of beef broth and 2 sprigs of fresh thyme. Bring the soup to a simmer and let it cook for at least 15 minutes to allow the flavors to meld.

Preheat your broiler and ladle the hot soup into oven-safe bowls, then top each with 2 slices of baguette and a generous sprinkle of shredded Gruyère cheese. Place the bowls under the broiler for 2-3 minutes, or until the cheese is bubbly and golden brown.