Preheat oven to 350°Lightly grease 9-inch baking pan. Fry the bacon in a skillet and remove to plate covered with paper towels to drain. Drain some of the bacon grease from the skillet, leaving a thin coating of bacon grease in the skillet for cooking the frittata in.
Heat bacon grease in large skillet over medium heat. If you used a cast iron skillet for the bacon, it should still be hot as cast iron retains heat for quite a while. Add zucchini, onion and parsley and saute until softened, about 4-5 minutes. Remove vegetables from skillet and set aside.
In large bowl, use a whisk or fork to beat eggs with cheese and seasonings. Crumble the bacon and add to the bowl (amount of bacon depends on your taste). Add the zucchini/onion mixture to the bowl. Blend everything together in the bowl with a fork or slotted spoon or whisk
Pour egg mixture into previously greased baking pan and bake until just firm in center, about 20-25 minutes. Serve hot or cold.