Slice onions thinly. In a large pot, melt butter with olive oil over medium heat. Add onions and cook slowly, stirring frequently, until deeply golden and caramelized, about 30-40 minutes.

Add white wine to deglaze the pan, scraping up browned bits. Stir in beef broth, thyme, salt, and pepper. Bring to a simmer and cook uncovered for 15 minutes to deepen flavors.

Slice baguette into 1/2-inch thick rounds. Toast until golden. Grate Gruyère cheese.

Ladle hot soup into oven-safe bowls. Top each with toasted bread slice and generous cheese. Place under a broiler until cheese is bubbly and golden, about 3-5 minutes.