Step 1

Place a large pot over medium heat and melt butter and oil together. Add onions, lower heat to low and cook down and caramelize for 20 minutes.

Step 2

Add the thyme sprigs and bay leaf and sprinkle onions with sugar, some salt and black pepper and continue to cook down for an additional 10 to 12 minutes.

Step 3

Add broth and sherry and bring the soup to a simmer. Simmer for about 15 minutes. Adjust seasonings.

Step 4

Discard the thyme stems and bay leaf and ladle soup into 3 oven safe bowls.

Step 5

Top each soup with 2 slices of bread and followed by 1 slice each Swiss and gruyere.

Step 6

Turn broiler on high. Place bowls of soup onto a baking sheet and broil for about 1 minute, or until the cheese begins to bubble and brown. Serve immediately.