Melt butter in a large pot over medium heat. Add the onions and cook gently.
At first it will look like too many onions but they will reduce significantly.
Continue to cook gently until they are a rich golden color.
Deglaze the pot with white wine. Add the broth, bay leaf, thyme, salt, and pepper. Cover and cook for 20 minutes.
Serve the soup, cover with bread and cheese. Broil until the cheese is melted and golden in spots.