Beat the eggs together in a large bowl, add the mozzarella cubes, the milk and the parsley and season to taste with salt and freshly ground black pepper.
Pour the oil into an 8in frying pan and heat until sizzling. Pour in the egg mixture and pull the mixture into the centre, rotating and turning the pan so that the mixture is allowed to set and brown on the under side.
After about 5 minutes, or when the mixture feels firm and reasonably set, turn the frittata over by placing a large plate on top of the pan and turning the pan upside down so that the frittata falls on to the plate with the cooked side uppermost.
Carefully slide the Frittata back into the hot pan with the cooked side on top and the uncooked side underneath.
Shake the frying pan to settle the contents and cook for a further 3 or 4 minutes.
Slide the frittata out of the pan on to a clean serving plate and serve hot or cold.