01 - Melt butter in a large skillet over medium heat. Add thinly sliced onions and cook, stirring frequently, until deep golden brown and caramelized, about 25 minutes.
02 - Add white wine and thyme to the pan. Scrape up any browned bits, then pour in beef broth. Season with salt and pepper. Simmer uncovered over low heat for 30 minutes.
03 - Preheat broiler. Spoon soup into oven-safe bowls, top each with a slice of French bread, and cover generously with Gruyere cheese. Broil until cheese is bubbling and golden brown, about 3 minutes.