Step 1

Preheat the oven to 400 degrees F.

Step 2

Heat a Dutch oven over medium heat and cook the bacon. Remove to a paper towel-lined plate to cool. Crumble the bacon into small pieces.

Step 3

Add the onions, sugar and 1 teaspoon salt to the bacon fat in the Dutch oven and cook over medium-high heat for about 5 minutes. Reduce the heat to medium-low and cook, stirring often, until the onions turn a deep golden-brown color, as long as 1 hour. Add up to 1/4 cup water or stock one tablespoon at a time if the pan gets too dry. (I usually add a bit of liquid about every 10 minutes.)

Step 4

Add the sherry or sherry vinegar and cook for 1 minute. Stir in the thyme and remove from the heat.

Step 5

Mix together the sour cream and mayo in a medium bowl. Add the crumbled bacon, Gruyere, onion mixture, black pepper and remaining teaspoon salt. Mix well and transfer to a 10-inch cast-iron skillet or baking dish. Top with 2 tablespoons Parmesan.

Step 6

Bake until bubbly and golden brown on top, about 20 minutes. Remove from the oven and top with the remaining 2 tablespoons Parmesan. Serve with toasted baguette slices for dipping.