In a large skillet, melt 2 tablespoons of butter with 1 tablespoon of olive oil over medium heat. Add 2 large sliced onions, 1 tablespoon of sugar, and a pinch of salt. Cook for about 25–30 minutes until golden brown.

Preheat your oven to 400°F (200°C).

Thaw and roll out puff pastry on a clean surface. Transfer to a parchment-lined baking sheet and score a 1-inch border. Prick the center with a fork.

In a mixing bowl, whisk together 1/2 cup of heavy cream, 1 large egg, 1 cup of shredded Gruyère, and a dash of black pepper. Optionally add a pinch of nutmeg.

Spread Dijon mustard on the puff pastry if desired. Pour the cheese mixture into the center and top with caramelized onions.

Bake for 15–18 minutes until the pastry is puffed and golden brown.

Garnish with fresh thyme and sea salt. Serve warm or at room temperature.