Preheat the oven to 350°F (175°C).
Prepare the crust: Line an 8" pie plate with your uncooked pie crust and set aside.
Cook the meat: In a skillet over medium heat, cook the breakfast sausage or bacon until fully cooked. Drain excess grease if needed, then set the meat aside.
Cook the onion and garlic: In the same pan, sauté the chopped onion until soft and translucent. Add the minced garlic and cook for about 30 seconds to 1 minute, just until fragrant. Remove from heat.
Make the egg mixture: In a large bowl, whisk together the eggs and milk until well combined. Stir in the shredded cheese and season with salt and pepper.
Combine everything: Add the cooked sausage or bacon and the onion/garlic mixture into the egg and cheese mixture. Stir to combine.
Fill the crust: Pour the mixture into the prepared pie crust, spreading the meat and cheese evenly.
Bake: Place the quiche in the oven and bake at 350°F for 55–60 minutes, or until the center is set.
Cool slightly and serve: Let the quiche rest for about 10 minutes before slicing. This helps it set up and makes it easier to cut clean slices.