Add onions, sugar, and salt to bacon fat in the skillet and cook over medium-high heat for about 5 minutes. Reduce heat to medium and cook, stirring often, until onions turn a deep golden brown color. This can take as long as 20 minutes. If onions start to burn, turn heat down to medium-low.
Add sherry and cook 1 minute. Stir in thyme and tabasco. Remove from heat.
Mix together sour cream and mayo in a medium bowl. Add crumbled bacon, cheese, onion mixture, and black pepper. Mix together well and transfer to a 2-cup baking dish.
Bake 20 minutes, or until bubbly and golden brown on top.