Preheat the oven to 375°F (190°C).
In a medium skillet, heat olive oil over medium heat. Add chopped onion and bell pepper and sauté for 5 minutes until softened. Remove from heat and set aside.
Place the unbaked pie crust into a 9-inch pie dish, pressing it firmly against the sides and bottom.
In a large mixing bowl, whisk together the eggs, low-fat milk, salt, black pepper, and garlic powder until well combined.
Layer the diced cooked chicken breast evenly over the pie crust.
Add the sautéed onion and bell pepper, and chopped spinach over the chicken layer.
Sprinkle the shredded cheddar cheese evenly on top of the vegetables and chicken.
Pour the egg mixture carefully over all the ingredients in the pie crust.
Bake the quiche in the preheated oven for 35 to 40 minutes, or until the center is set and the top is lightly golden.
Remove the quiche from the oven and let it cool for 10 minutes before slicing and serving.