Add butter to a large pan, fold in onions and slowly saute until carmelized under low to medium heat. Caramelizing onion takes time and patience. Let the onions sit and stir occasionally to avoid sticking or burning. The onions should have a thick and syrupy consistency when ready, approximately 20 minutes.

Once the onions are caramelized add wine and bring up to a low simmer for five minutes. Slowly fold in the flour to thicken the mixture.

Warm broth over low heat in a separate pot and add to the onions slowly. Add bouquet garni, a pinch of salt and pepper, and bring to a low simmer.

Prepare the oven broiler. Cut four slices from the baguette. Place four oven-proof containers on a baking sheet. Fill the containers with soup and add bread slices atop each container. Top each with grated Gruyère and broil in the oven for three and four minutes. Serve warm.