Step 1

Cook the bacon, stirring often, in a large non-stick fry pan over medium heat until almost crisp. Add the onions, sugar and salt and cook, stirring often until they cook down by two thirds.

Step 2

Add the 2 teaspoons vinegar, reduce heat to medium-low, and continue to cook until the onions are an even golden brown and softened, 15 - 20 minutes. (depending on the onions)

Step 3

Stir often and scrape up any brown bits clinging to the bottom of the pan. Remove from heat, stir in the thyme and allow the ingredients to cool slightly. Taste, season with black pepper and add more salt if necessary.

Step 4

Preheat oven to 350 degrees. Stir the cream into the caramelized onions until thoroughly combined.

Step 5

Place the filling in a 9" tart shell that's been blind baked and spread it evenly with the back of a spoon. Sprinkle grated gruyere on top. Bake in oven until the filling is set, about 15 minutes.

Step 6

Serve warm or at room temperature. Garnish with a sprig of thyme.