In a large heavy skillet, cook the bacon until crisp and all fat has been rendered. Set bacon aside to drain on paper towels. Pour off the rendered bacon drippings, then return 3 tablespoons of the drippings to the hot pan. (Remaining drippings can be saved for another purpose.)

Add the onions to the hot pan and season with kosher salt and a generous amount of black pepper. Add the thyme. Cook over medium to medium-low heat, stirring occasionally but more frequently as the cooking progresses, until the onions are completely translucent and caramelized to a deep golden color, 15 to 20 minutes.

Add the brown sugar, stirring until the sugar melts. Add the Port and continue to cook until the liquid has evaporated and onions are shiny and glazed, 2 to 3 minutes longer.

Taste and adjust the seasoning if necessary. Allow to cool to room temperature.

The reserved bacon slices may be saved for another purpose, or used to garnish the burgers, as desired.