in a large frying pan over medium-low heat, fry the bacon until renders its fat and is lightly browned but not too crisp, 4 or 5 minutes. Using a slotted spoon, transfer to the bacon to a paper towel lined plate. Now add the onions and toss to coat with the bacon fat. Add the wine, thyme salt and pepper and stir to combine. Cover and cook, stirring once or twice, until the onions are very soft and lightly browned, about 25 minutes. Uncover, increase the heat to medium, and continue to cook, stirring occasionally until any liquid has evaporated, 3-4 minutes. The onions should be quite dry. Transfer the onions to a bowl and let stand until slightly cooled, about 10 minutes.

Meanwhile preheat the oven to 375 degrees. Roll out the puff pastry on a sheet of parchment paper unto an 11 inch square and 1/8 inch thick. Transfer the parchment and pastry to a baking sheet. Turn up the pastry edges about a half inch to make a small raised edge. Prick the pastry all over with the tines of a fork. Bake the pastry until it just starts to color, but not brown, and is dry to the touch, about 10 minutes. Remove from the oven.

Raise the oven temperature to 400 degrees. Add the Gruyere cheese and the reserved bacon to the cooled onions and mix well. Sprinkle the pastry with the Parmesan cheese, and then spread the onion mixture evenly over the pastry, getting as close to the edge as you like. Bake the tart until the edges of the pastry are lightly browned a few small brown specks have appeared on the onions, about 20 minutes. Let the tart cool slightly.

Scatter additional thyme leaves over the top, if you like.

Using a sharp knife or a pizza wheel, cut into squares that suit your needs. Serve warm or at room temperature.