Preheat your oven to 375°F (190°C) and grease a 9-inch deep dish pie plate with butter.

Fry the chopped bacon in a skillet over medium heat until golden brown and crispy, about 5-7 minutes. Drain the excess grease.

Sauté the chopped yellow onion in the bacon grease over medium heat for 7-8 minutes until soft and translucent.

In a mixing bowl, whisk together the eggs, half and half, kosher salt, cayenne pepper, and black pepper. Fold in the bacon, sautéed onion, and Gruyere cheese.

Pour the custard mixture into the prepared pie plate, sprinkle with Parmesan cheese and chives, then bake for 35-40 minutes until golden and slightly jiggly in the center.

Let the quiche cool for about 10 minutes before slicing. Serve warm or at room temperature.