Peel and chop the potatoes into 1-inch large cubes.
In a large pot of lightly salted boiling water, boil the potatoes for about 5 min, until just tender.
In the meantime, in a large bowl whisk the eggs until well combined, add in chopped parsley and season with salt and pepper.
Drain well the potatoes and add them into the bowl with the eggs. Fold in the cheese (if using) and lightly mix all ingredients.
Heat a large non-stick skillet with the olive oil over medium-low heat. Pour in the frittata mixture, making sure to distribute the potatoes evenly over the bottom of the pan.
Cover with a lid and cook for about 5-10 minutes, gently shaking the pan every now and then. When the frittata is easily pulled off from the bottom, is ready to be turned. Using a plate with the same diameter of the frittata, carefully put it on top and flip the frittata on the other side.
Cook on the other side for a further 2-5 minutes, this time without the lid.
When the frittata is fully cooked, remove from the pan and slide it gently onto a serving plate, and serve
Store leftovers in an airtight container and refrigerate for up to 2 days.