Slice the onions thinly and melt butter in a large pot or Dutch oven over medium heat.

Add the sliced onions to the pot and cook, stirring occasionally, for 30-40 minutes until deeply caramelized and soft.

Pour in the dry white wine and scrape up any browned bits from the bottom of the pot.

Add the beef broth, salt, and pepper; bring to a simmer and cook for 20 minutes.

Preheat your oven broiler and slice the baguette into 1-inch thick rounds.

Ladle the soup into oven-safe bowls, float a baguette slice on top, and sprinkle generously with shredded Gruyere cheese.

Broil for 2-3 minutes, watching carefully, until the cheese is bubbly and golden brown.