Peel and slice the onions thinly.

In a large pot over low heat, melt the butter and add the sliced onions. Cook slowly, stirring occasionally, until deeply caramelized, about 2 hours or more.

Ensure the onions are nicely browned and sweet; take your time to develop flavor.

Deglaze the pot with the red wine, scraping up browned bits from the bottom.

Add the beef stock, bay leaves, and thyme. Bring to a simmer, cover, and cook for 1 hour.

When ready to serve, preheat the oven broiler to high.

Ladle the soup into oven-safe bowls. Place a toasted baguette crouton on top of each bowl and pile generously with grated Swiss/Gruyère cheese.

Place the bowls under the broiler until the cheese is melted, browned, and bubbling.

Let the soup sit for about 10 minutes before serving. Garnish with fresh thyme or chives.