Preheat your oven to 375°F (190°C). Place the pie crust into a 9-inch pie dish. Press it gently into the edges. Prick the bottom with a fork. Bake for about 8 minutes to lightly pre-bake.

Place bacon slices in a frying pan over medium heat. Cook slowly until crispy and golden. Remove the bacon and place on paper towels. Once cooled, chop the bacon into small pieces.

In the same pan, add butter and chopped onions. Cook them over medium heat. Stir occasionally until onions become soft and slightly golden. This step adds a sweet and rich flavor.

In a large bowl, crack the eggs. Add heavy cream and milk. Whisk until smooth. Add salt, black pepper, and nutmeg. Keep whisking until everything is well combined and slightly frothy.

Spread the cooked onions evenly across the crust. Sprinkle the chopped bacon on top. Add shredded Swiss cheese evenly. This layering helps the flavors mix beautifully while baking.

Slowly pour the egg mixture into the crust. Pour gently so ingredients stay evenly spread. The liquid should fill most of the crust but not overflow.

Place the pie dish on a baking sheet. Carefully place it in the oven. Bake for about 35–40 minutes. The center should be set but slightly soft.

Remove the quiche from the oven. Let it rest for about 10 minutes. This helps the filling set properly. Slice carefully and sprinkle fresh parsley if desired.