Preheat the oven to 350°F (175°C).
In a skillet, heat the olive oil over medium heat.
Add the diced bell peppers to the skillet and sauté for 3-4 minutes until slightly softened.
Stir in the chopped spinach and cook for an additional 2 minutes, allowing it to wilt.
In a bowl, whisk together the eggs, milk, salt, pepper, and garlic powder until well combined.
Pour the egg mixture over the sautéed vegetables in the skillet.
Sprinkle the shredded cheddar cheese on top of the egg mixture.
Transfer the skillet to the preheated oven and bake for 20-25 minutes, or until the frittata is set and lightly browned on top.
Remove from the oven and let it cool for a few minutes before slicing.
Garnish with fresh herbs before serving.