Preheat oven to 375 degrees F (190 degrees C).

Melt the butter in a large skillet over medium-high heat.

Add the sliced onion to the skillet and sauté for 5-10 minutes, until tender and slightly caramelized.

Stir in the dried thyme and remove from heat.

In a large pot, combine the beef consommé and white wine (if using).

Bring to a boil over high heat, then reduce heat to low.

Stir in the sautéed onions and simmer for 10-15 minutes to allow flavors to meld.

Ladle the soup into 4 individual oven-safe bowls, filling each about 3/4 of the way.

Top each bowl with 2 slices of baguette bread.

Cover each baguette with shredded Swiss cheese.

Bake in the preheated oven for 10-15 minutes, or until the cheese is melted, bubbly, and lightly browned.