1. Preheat oven to 425 degrees Fahrenheit.
2. In a 12-inch cast iron skillet, sauté onion and peppers in a bit of oil until mostly soft. Add zucchini and sauté for three more minutes.
3. In a bowl, whisk eggs with milk, 1/2 cup cheese, salt, pepper, and garlic powder. Pour over the vegetables. Cook on medium heat for one to two minutes until edges begin to brown.
4. Pour marinara sauce over entire frittata and cover with remaining cheese.
5. Transfer skillet to the oven and bake for 10–12 minutes. Keep an eye on the frittata to make sure it doesn’t burn.
6. Serve immediately.