Step 1

Preheat the oven to 350 degrees F (175 degrees C).

Step 2

Fry bacon in a large skillet over medium-high heat, turning occasionally, until crispy, about 10 minutes. Drain bacon slices on paper towels. Let cool 10 minutes. Crumble and set aside.

Step 3

Roll out pastry crust into a circle using a lightly floured rolling pin. Line a 9-inch pie pan with crust. Prick holes into the base of the dough using a fork.

Step 4

Bake crust in the preheated oven on the middle rack for 15 minutes.

Step 5

Remove pastry crust from the oven. Brush crust with beaten egg. Return crust to the oven and bake 5 minutes more.

Step 6

Heat butter in a pan over medium heat and stir in onion. Cook and stir until onion has softened and turned translucent, about 5 minutes. Reduce heat to low and continue cooking and stirring until onion is very tender and dark brown, about 40 minutes more.

Step 7

Whisk 2 eggs together in a bowl using a fork. Add sour cream and cream cheese and beat using an electric mixer until well combined. Beat in salt and pepper.

Step 8

Spread caramelized onions with remaining butter onto pastry crust. Sprinkle with 1/2 the Cheddar cheese. Place crumbled bacon evenly over the top. Pour egg mixture over bacon. Top with remaining Cheddar cheese.

Step 9

Bake quiche in the preheated oven until golden and fluffy, about 30 minutes. Let cool 10 minutes before serving.