Heat Oil and Onions: Heat a little olive oil in a cast iron pan (you can also use an oven-safe skillet or casserole dish) over medium heat and add the onion, garlic and zucchini.
Combine Eggs With Other Ingredients: Pour the egg mixture over the cooked vegetables and crumble feta or mozzarella cheese over the top in a baking dish.
Bake: Place your cast iron skillet directly in the oven. Bake for 25 minutes at 375 F / 180 C until the frittata is set in the middle and slightly golden on top.