Heat oil on medium heat in large flame-proof frying pan. Add onion and saute for 5 minutes until softened, stirring occasionally. Add potatoes and cook for a further 5 minutes until golden, stirring to prevent sticking. Spread mixture evenly over bottom of pan. Preheat grill to high, or turn on oven broiler. Pour beaten eggs, lightly seasoned with salt and pepper, over potato and onion mixture. Sprinkle the cheese on top and cook over medium heat for 5-6 minutes until the eggs are just set and base of frittata is lightly golden. Place pan under grill or broiler (protect the pan handle with a double layer of foil if it is not flameproof) and cook the top of the frittata for about 3 minutes until it is set and lightly golden. Serve warm or cold, cut into wedges.
Try served with crusty bread and a tomato salad! Yum!