Preheat your grill to medium-high heat.

Using a non-stick frying pan that you can put under the grill, heat the oil over a medium heat.

Add the onion and cook until just starting to brown. Add the spinach and toss for a minute or two until it begins to wilt. Remove from the heat and allow to cool.

Beat the eggs in a bowl. Add in the cooled spinach and onion, and then the feta. Season to taste.

Put your frying pan back on a medium heat and add the eggs. Stir gently with a spatula until you feel the egg start to set on the bottom. Turn off the heat so the frittata stays quite runny.

Place your frying pan under the grill for 2 to 3 minutes, or until the frittata is golden and cooked all the way through (check with a fork.)

Place a plate over the pan and turn over quickly but carefully to release the frittata. Serve hot or cold with a crispy side salad.