Slice the onions thinly and melt butter in a large pot or Dutch oven over medium-low heat. Add the sliced onions to the pot and cook slowly, stirring occasionally, for 30-40 minutes until they are deeply golden brown and caramelized.

Pour in the dry white wine and scrape up any browned bits from the bottom of the pot. Add the beef broth, bring to a simmer, then reduce heat and let it gently simmer for at least 20-30 minutes for the flavors to meld.

Preheat your oven to 400°F (200°C) and slice the baguette into 1-inch thick rounds. Ladle the hot soup into oven-safe bowls, top each with a baguette slice, and sprinkle generously with shredded Gruyere cheese. Bake for 10-15 minutes, or until the cheese is melted and bubbly and the croutons are golden brown.