To wilt the spinach, place into a colander and slowly pour over a kettle of boiling water. Next, run a cold tap over the spinach until cooled. Drain, then squeeze spinach to remove any excess water.
Pre-heat grill to medium. Heat oil in a non-stick pan and gently fry the onion and potato for around 15 minutes, until the potato is soft.
Meanwhile, beat the eggs in a large bowl with a generous grind of Parsley, Salt and Pepper. Add the wilted spinach to the pan, then pour over the eggs. Stir the pan until the eggs begin to set then allow to cook for a few minutes, until the underside is cooked.
Finish the omelette under the grill for a few minutes until set, then transfer to a board and cut into slices. Garish with an extra grind of Parsley before serving, and have on hand at the table to grind on extra flavour to taste.