Lightly dust flat surface with flour and place 2 sheets of puff pastry on it. Place one edge over the other and roll it out so the edges are rolled into the dough.
Gently lay the pastry over the pie dish and use your fingers to fill-in all crevices and corners. Trim away excess dough so the dough is sticking just a little bit above the dish. Use a fork to poke the bottom of the dough about 7-10 times.
Heat a small frying pan to medium heat and melt the butter in it. Add in the chopped onions and cook for about 5 minutes. (They should be translucent but not caramelized or burnt).
Chop the cooked bacon and grate the Swiss cheese and set to the side.
In a large mixing bowl combine the eggs, whipping cream, salt, pepper, nutmeg and cayenne pepper. Beat everything together and set tot he side.
Spread cooked onions in a layer at the bottom of the crust. Use half the bacon to spread a layer on top of the onions. Use half the shredded cheese to spread a layer on top of the bacon and repeat with the rest of the bacon and cheese.
Slowly pour the egg mixture into the dish so that it spreads out evenly throughout the dish.
Place the dish in the oven and bake it for 15 minutes.
Reduce heat to 300°F and bake 30-45 minutes. It’ll be done when you can stick a knife in and it’ll come out clean.