1. Heat the oil in a 22 - 24cm square frying pan.
2. Add the onions, ensuring that the slices are sperated and fry gently for 4 -5 minutes until beginning to brown.
3. Add the potatoes and saute for another 4 - 5 minutes to brown slightly.
4. Preheat the grill on medium.
5. Pour over the beaten eggs evenly. Tilt the pan to ensure that egg runs into and down the edges.
6. Put the pieces of Feta on the top of the egg to evenly spread over the whole surface.
During cooking, lift the pan occasionally to float more egg to the edges.
Test that the base is browning and setting by gently priming the edge away from the pan with knife. When the base is done, place the whole pan under the grill and cook until top is golden brown and cooked all over. If one part browns quicker than others, cover this part with a piece of kitchen foil until evenly golden.
7. Leave in the pan for 5 - 10 minutes to set and cool slightly. Carefully slide the frittata out of the pan onto a serving dish/plate.
Eat hot, or cold as a pack lunch/picnic ingredient.