Begin by cutting the bacon into bite-sized pieces and frying them in a skillet over medium heat. Cook for about 8–10 minutes until crispy and golden brown. Use a slotted spoon to transfer the cooked bacon onto paper towels to drain excess fat, leaving a little fat in the skillet for added flavor in the next step.

Reduce the heat to medium-low and carefully pour off excess bacon fat, leaving about a tablespoon in the skillet. Add the chopped yellow onion, sautéing for 7–8 minutes until they turn translucent and tender.

In a large mixing bowl, whisk together the large eggs, half and half, kosher salt, cayenne pepper, and freshly cracked pepper until well combined. Fold in the sautéed onions and crispy bacon, followed by the shredded Gruyere and Parmesan cheeses, ensuring everything is evenly mixed.

Preheat your oven to 375°F (190°C). Grease a 9-inch deep dish pie plate and pour the quiche mixture into it. Sprinkle additional Parmesan cheese on top, along with some chopped chives. Bake for 35–40 minutes until golden brown and set in the center.