In a large skillet, melt 2 tablespoons of butter and 1 tablespoon of olive oil over medium heat. Add 3 thinly sliced onions, 1 tablespoon of sugar, and 1 teaspoon of salt. Cook slowly for 25-30 minutes, stirring occasionally, until the onions are golden brown and jammy in texture.

While the onions are caramelizing, preheat your oven to 400°F (200°C).

Lay your thawed puff pastry on a parchment-lined baking sheet. Score a 1-inch border around the edges and gently prick the center with a fork.

In a medium bowl, whisk together 1/2 cup of heavy cream, 1 beaten egg, 1/4 teaspoon of black pepper, and 1 cup of shredded Gruyère cheese.

If using Dijon mustard, spread a thin layer on the pastry base. Pour the cheese mixture onto the pastry and top with the caramelized onions.

Place the assembled tart in the preheated oven and bake for 15-18 minutes.

Once baked, remove from the oven, allow to cool slightly, then garnish with fresh thyme and a sprinkle of sea salt.