Preheat your oven to 375°F (190°C).

Chop the fresh spinach into small pieces. If using frozen spinach, thaw and squeeze out excess water.

In a large mixing bowl, whisk together the eggs and whole milk until well combined. Add the shredded cheddar cheese.

Add the chopped spinach, diced onion, and diced bell pepper to the egg and cheese mixture. Sprinkle in garlic powder, salt, black pepper, and red pepper flakes. Stir until evenly mixed.

Pour the mixture into a greased 9-inch pie dish or quiche pan, spreading it out evenly.

Bake in the preheated oven for 30-35 minutes until the center is set and the top is lightly golden. Let cool for 5-10 minutes before slicing.