Preheat oven to 375 degrees Fahrenheit.
In a medium size bowl crack eggs. To the eggs add milk, salt, pepper, chili flakes, and Italian seasoning. Whisk to combine. Add parmesan and 1/2 cup of the mozzarella cheese to the egg mixture and whisk to combine. Set mixture aside.
In a 10 inch cast iron pan add the olive oil and set to medium heat until the olive oil is simmering. Add to the pan the Italian sausage, bell peppers, red onion to pan and cook with the Italian sausage for 3-5 minutes, stirring occasionally. Next add the mushrooms and pepperoni to the pan and cook for 3-5 minutes. Add the black olives and spinach to the pan and stir to combine with other ingredients in the pan.
Whisk the egg mixture again and then pour into the cast iron. Use a spatula to distribute the egg mixture evenly in the pan.
Keep the pan on the stovetop on medium heat for 2-3 minutes, until the edges of the frittata turn lighter in color. Transfer the pan carefully to the oven and bake for 7-10 minutes. Once 7-10 minutes have passed and the top of the frittata is slightly cooked, add the rest of the mozzarella cheese to the top of the frittata and turn the oven on broil for 2-3 minutes or until the cheese on top is golden brown.
Remove the frittata from the oven and let rest for 5-10 minutes. Sprinkle with chili flakes, cut with a sharp knife, and serve.