Peel potatoes, slice very thinly and set aside. Slice onions very thinly.
Heat oil in a 10 inch nonstick, oven-proof skillet. Layer potato slices in pan and cook over medium heat for 5 minutes; lower heat if potatoes begin to brown. Using a pancake turner, turn potato slices over, sprinkle onions on top and cook for 5 more minutes. Repeat 2 more times, turning onion with potato, until potatoes have cooked for 20 minutes total. Onions will be translucent and potatoes will be soft and slightly golden.
Beat eggs with a whisk in a large bowl. Add rep pepper, salt and pepper, beat again and pour mixture into skillet. As egg begins to cook and become firm around the edges, carefully lift edge with a flexible spatula, allowing uncooked egg to slide under the cooked egg.
Cook for 5 minutes on top of the stove. Then sprinkle with Parmesan cheese, place skillet in oven and bake at 350 degrees for 15 minutes, until golden.
Loosen frittata from skillet edge with spatula and slide onto a large serving plate. Cut into wedges and serve. Frittata can be enjoyed either warm or at room temperature.