In a large skillet, add butter and olive oil on medium high heat. When the butter is nearly melted and starting to get bubbly, add in the sliced onion.

Season the onion with 2 teaspoons of salt and give it a good stir lower the heat to medium low heat and allow the onions to caramelize, this process should take at least 20 minutes.

Stir every few minutes on medium low heat. Next add in the anchovies the mustard the fresh thyme, and sugar. Stir together and continue to cook on medium low heat for another 10 minutes.

Once the puff pastry has defrosted place it on a baking dish or baking sheet and gently press, the puff pastry to spread a bit.

Add the peach or apricot jam to the center of the puff pastry and spread it out with the back of a spoon, leaving about an inch border.

Add half of the graded Gruyere cheese and top with the onion mixture. Sprinkle the remaining cheese on top and garnish with a few sprigs of fresh thyme.

Fold the 1 inch border sides of the puff pastry in and press down. bake the onion tart at 400° for 20 to 25 minutes or until golden and bubbly.