Preheat your oven to 375°F; once hot, place a well seasoned and oiled 10" cast iron skillet on the center rack to preheat while you prep your ingredients.
In a large mixing bowl, combine the eggs, milk, salt, pepper and chili pepper flakes (if using) and beat until slightly frothy. Add the grated zucchini and cheese and stir delicately until well combined.
Carefully take your cast iron skillet out of the oven and pour the frittata mixture in it.
Return the pan to the oven and bake for 20 minutes (25 minutes if you're not going to garnish the top), then take the frittata out of the oven and arrange thin slices on top of the frittata and sprinkle with the grated cheese.
Return the pan to the oven and bake for a further 8 to 10 minutes, until the cheese is melted and starts to brown. Set the oven to broil and leave the frittata under it for 2-3 minutes if further browning of the cheese is desired.
Remove the frittata from the oven and let it rest for about 5 minutes, then garnish with a little bit of freshly chopped parsley and ground black pepper, if desired, and serve.
*Place the grated zucchini in a clean dish towel or better yet, a nut bag, and squeeze to remove as much excess water as you can.