Boil the sliced potato for 10 minutes until just tender, then drain and set aside.
Meanwhile, beat the eggs in a large bowl. Heat the oil in a 22cm ovenproof frying pan and cook the red onion for 5 minutes, until softened.
Stir the potatoes, tomatoes and frozen peas into the beaten eggs. Season with freshly ground black pepper.
Pour the mixture into the frying pan on top of the cooked onions. Cook over a gentle heat for 15 minutes. After 10 minutes, preheat the grill to hot.
Scatter the cheese over the top of the frittata and place under the grill for 5 minutes, until golden on top, set and cooked through. Cut into wedges and serve with salad.