1Heat the broiler on high and arrange a rack in the middle.
2Whisk the eggs, 1 cup of the cheese, and the measured salt and pepper in a large bowl until the eggs are smooth; set aside.
3Heat the oil in a large, ovenproof frying pan over medium heat. When it shimmers, add the onion and season with salt and pepper. Cook, stirring occasionally, until tender, about 4 minutes. Add the potato and cook, stirring occasionally, until the edges become tender, about 4 minutes. Add the bell peppers and garlic and cook, stirring occasionally, until just tender, about 8 minutes. Add the parsley and stir to incorporate.
4Pour the reserved egg mixture over the vegetables and reduce the heat to medium low. Cook until the eggs are set around the outer inch of the pan, about 10 to 15 minutes. Sprinkle the remaining cheese evenly over the frittata and place it under the broiler until puffed, golden, and completely set in the center, about 4 to 6 minutes. Let sit 5 minutes before serving.