Preheat the oven to 375°F.
In a skillet, heat olive oil over medium heat. Add diced onion and bell pepper, sautéing until softened, about 5 minutes.
Stir in the chopped spinach and cook for an additional 2 minutes until wilted. Remove from heat and let cool slightly.
In a large mixing bowl, whisk together milk, eggs, garlic powder, salt, and black pepper until well combined.
Stir in the sautéed vegetables and shredded cheddar cheese.
Roll out the pie crust on a lightly floured surface and cut out circles to fit into a greased muffin tin (about 3 inches in diameter). Press the circles into the muffin tin cups.
Pour the egg and vegetable mixture into each crust, filling them about 3/4 full.
Bake in the preheated oven for 20-25 minutes, or until the quiches are set and lightly golden on top.
Allow to cool for a few minutes before removing from the muffin tin.