Gather all ingredients. Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9-inch pie pan.
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes.
Drain bacon slices on paper towels. Crumble and set aside.
Cover bottom of prepared pie pan with shredded cheese and crumbled bacon.
Whisk together milk, eggs, flour, onion, butter, and salt in a large bowl until smooth. Pour mixture over cheese and bacon in pie pan.
Bake quiche in the center of the preheated oven until golden brown and firm in the center, about 35 minutes. Serve hot or cold.
