A frittata is an Italian omelette: thick, unfolded and cooked on one side on the hob, then, to make sure the whole's cooked through, placed under the grill for a few minutes. It tastes best cold. Not fridge-cold, just completely cool.
Put the peas in a bowl, pour over boiling water and leave until you're ready to add them to the frittata. If you're using fresh peas, cook them in a boiling water for 15 min.
Preheat the grill. Heat the oil in a 25cm, non-stick, ovenproof frying pan and fry the onion until it begins to soften.Add the drained peas. Transfer the mixture to a large bowl to cool slightly.
Break the eggs into the bowl and beat. Season with salt and pepper and add the parmesan and previously cooked vegetables.
Put the pan back on the heat. Pour the egg mixture into the pan and cook over medium heat for 8 minutes, or until the omelette is firm. Remove the pan from the heat and place it under the grill until the top of the omelette is golden, and it begins to swell again. Let cool, then cut into wedges.
Serve with sliced mozzarella, a few drops of olive oil and pesto and a few slices of Parma ham.