The next step is to halve and slice the unpeeled potatoes. We also peel, halve, and slice the piece of onion thinly.
We pre-heat the oven to 400° F.
We heat up our 10 inch non-stick pan (a pan that is oven-safe) with the olive oil over medium heat. We add the onions and cook, stirring with a heat-safe spatula for a few minutes until tender. Next, the basting oil is added, and when hot, we add the potatoes. We try to cook the potatoes in one layer, and, when one side is browned, we flip the potatoes over.
Meanwhile, we beat the eggs, salt, pepper, Locatelli cheese, sage and chives in a medium bowl with a fork. When the potatoes are nicely browned, we pour the egg mixture into the pan and lower the heat to medium-low. We stir gently to distribute the potatoes.
After a couple of minutes, the eggs are starting to set. We place the pan in the oven and cook for another 5 minutes. With a potholder, we take the frittata out of the oven. Using our rubber spatula, we check that it is loose enough on the bottom. We place the mozzarella on the top, both for flavor and decoration. The pan goes back in the oven for another 5 minutes to melt the cheese.
Carefully, we slide the frittata onto a clean cutting board, sprinkle with the paprika, cut into wedges and enjoy.