Step 1

Preheat the oven to 400°F.

Step 2

In a large, heavy soup pot or Dutch oven, melt the butter over medium-low heat. Add the onions and cook, covered, for 20 minutes. Place the soup pot into the oven with the lid slightly ajar to ensure the onions will brown. Allow the onions to cook in the oven for 1 hour, stirring once, half way through, so the onions don't stick and burn.

Step 3

Remove the pot from oven and place back on the stovetop over medium heat. If the onions aren't yet golden brown, cook for 5 to 10 minutes more, stirring frequently and scraping off all the brown, flavorful bits, until golden.

Step 4

Turn off the heat and pour in the wine. Return the heat back to medium. Cook the wine for 5 minutes, allowing it to reduce. Add the broths, Worcestershire sauce, garlic, salt, and pepper. Bring to a simmer, then reduce the heat to low. Simmer for 30 to 45 minutes.

Step 5

Preheat the broiler to low. Butter one side of the bread slices and broil for 1 to 2 minutes, allowing the bread to brown and become crispy.

Step 6

When the soup is ready, ladle it into oven-proof bowls or ramekins set on a sheet tray. Place the crispy bread on top, then sprinkle generously with the gruyère. Broil until the cheese is melted and bubbly, 2 to 3 minutes. Serve immediately.