Preheat the oven to 400 °F.

Put the potatoes in a non-stick skillet on the stove and cover with cold water. Turn the heat to medium-high and cook about 15 minutes until the potatoes are easily pierced with a knife.

Drain the potatoes well and set aside to cool. Then peel and cut into 1-inch cubes.

Put the eggs, salt, and pepper in the bowl and mix well with a whisk.

Return the skillet on the stove and turn the heat to medium-high. Add the oil. Add onion and cook about 5 minutes until soft. Add the cubed potatoes.

Pour the egg mixture over the potatoes and onions. Press down with a spatula to make an even layer and shake gently to prevent sticking. Lower to medium heat and cook about 7 minutes.

Transfer the skillet to the oven and cook until the frittata is completely set, about 5 minutes.

Gently loosen frittata from the pan. Place a serving plate over the skillet and carefully invert the frittata onto the plate.