Step 1

For the soup:  Preheat the oven to 400 degrees F. Slice the onions in half from root to tip, then slice them thin.

Step 2

Melt the butter in a heavy ovenproof pot or Dutch oven over medium-low heat. Add the onions, stir to coat them in the butter and cook, covered, until translucent and soft, about 20 minutes.

Step 3

Transfer the pot to the oven with the lid slightly ajar. Roast the onions for 1 hour, stirring twice to keep them from burning. (Some brown bits on the bottom of the pan are fine though.)

Step 4

Return the pot to the stovetop over medium heat (keep the oven on) and pour in the wine. Stir, scraping the pan. Cook until the wine reduces, 5 minutes. Next, add the beef broth, chicken broth, thyme, Worcestershire sauce and garlic. Reduce the heat to low and let the soup simmer for 45 minutes.

Step 5

For the croutons: Meanwhile, drizzle slices of French bread with olive oil and toast in the oven until golden. Set aside.

Step 6

To serve, turn on the oven broiler. Ladle the soup into ovenproof bowls. Place 1 or 2 pieces of the croutons on top of each serving of soup, depending on the size of the bowl. Place a thick slice of cheese on top of each bowl, then place the bowls on a baking sheet and broil just long enough for the cheese to become melted, bubbly and slightly toasted.

Step 7

Serve immediately.