Preheat your oven to 425°F (220°C).

Prepare a 9-inch deep-dish pie plate by greasing it with butter or cooking spray.

In a frying pan, melt a tablespoon of butter over medium heat, add chopped onions, and sauté for about 5-7 minutes until soft and golden.

In a large mixing bowl, whisk together 4 eggs, 1 cup of whipping cream, and your choice of spices until well combined.

Unroll puff pastry on a floured surface and roll it out to fit your pie dish. Place it into the prepared pie plate.

Layer the sautéed onions, chopped bacon, and grated Swiss cheese evenly in the pastry.

Pour the egg and cream mixture over the layered filling, filling the dish just below the top of the pastry crust.

Bake for 15 minutes at 425°F (220°C), then reduce the temperature to 300°F (150°C) and continue baking for 35-40 minutes, until the filling is set.

Let the quiche cool for 15 minutes before slicing and serving.

Serve warm, optionally with a light salad or roasted vegetables.