Preheat the oven to 375°F (190°C).
Chop the onion, bell peppers, and spinach; set aside.
In a large mixing bowl, whisk together eggs and milk. Add in garlic powder, chili powder, salt, and pepper. Stir in the shredded cheese, chopped spinach, and diced vegetables.
In the baking dish, layer corn tortillas on the bottom, overlapping them if needed.
Pour the egg mixture over the tortillas evenly.
Bake for 30-35 minutes, or until the quiche is set in the center and slightly golden on top.
Allow to cool for 5-10 minutes before slicing; garnish with fresh cilantro and optional sour cream.