Preheat the oven: Set your oven to 375°F (190°C) and grease a muffin tin.
Cut the pie crust: Roll out the pie dough and use a round cutter or a glass to cut circles slightly larger than the muffin cups.
Press into the muffin tin: Gently press each dough circle into the muffin cups, making sure the crust fits snugly along the edges.
Prepare the egg mixture: In a bowl, whisk together the eggs, milk, salt, and pepper until well combined.
Add fillings: Sprinkle cheese and any mix-ins (ham, spinach, mushrooms, etc.) into each crust-lined muffin cup.
Pour in the egg mixture: Carefully pour the egg mixture over the fillings, filling each cup about ¾ full.
Bake: Place the muffin tin in the oven and bake for 18-20 minutes, or until the eggs are set and the tops are lightly golden.
Cool and serve: Let the mini quiches cool for 5 minutes before removing them from the pan. Serve warm and enjoy!