Preheat the oven to 375°F (190°C).
Lightly grease a 24-cup mini muffin pan.
Unroll the pie crusts and cut approximately 2 1/2 to 3-inch circles from the pastry.
Gently press one pastry circle into each mini muffin cup.
Finely chop the spinach, bell pepper, and green onions.
Whisk the eggs, milk, garlic powder, salt, and black pepper together in a medium bowl.
Divide the spinach, bell pepper, green onions, and shredded cheddar between the pastry shells.
Carefully pour or spoon the egg mixture into each shell, filling approximately three-quarters full.
Bake for 18–22 minutes, until the pastry edges are golden and the egg centers are set.
Allow the mini quiches to cool in the pan for approximately 5 minutes.
Carefully remove and serve warm or at room temperature.