Whisk eggs, salt, pepper, parsley, Parmesan cheese (if using) and garlic in a large bowl. Stir in the zucchini.
Heat the remaining olive oil in a 9-inch ovenproof skillet over medium-high setting. Add zucchini-egg mixture and cook without stirring until bottom and sides begin to set, about 4 minutes. Transfer skillet to oven and bake until eggs completely set and top is golden brown, 15 to 20 minutes.
To serve, set a plate on top of skillet (the plate’s underside should face up) and, holding plate to skillet with both hands, gently flip over. Lift off skillet. Slice frittata into wedges and serve.