In a large skillet, melt 1 tablespoon of butter with 2 tablespoons of olive oil over medium heat. Add 4 large, thinly sliced onions, 1 teaspoon of sugar, and salt to taste. Cook for 25-30 minutes, stirring occasionally, until the onions are golden, soft, and jammy in texture.

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

Once the onions are ready, roll out the thawed puff pastry on a parchment-lined baking sheet. Score a 1-inch border around the edge.

In a mixing bowl, whisk together 1 cup of heavy cream, 1 beaten egg, ½ teaspoon of black pepper, and 100 grams of shredded Gruyère cheese.

Spread a thin layer of Dijon mustard across the inner section of the prepared puff pastry. Carefully pour the cream mixture and top with caramelized onions.

Place the assembled tart in the preheated oven and bake for 15-18 minutes, or until the pastry is puffed and golden brown.

Once baked, remove the tart from the oven and let it cool slightly. Garnish with fresh thyme and a sprinkle of sea salt.