2. Preheat your broiler to high. Pour enough oil into a small 7 to 8 inch ovenproof non-stick frying pan to very lightly coat the bottom (about 1 teaspoon) and put over a medium heat.
3. Add the onion and cook until just starting to brown. Add the spinach and toss for a minute or two to heat through, then remove from the heat and allow the vegetables to cool slightly.
4. Beat the eggs in a bowl. Add in the cooled spinach and red onion, followed by the feta and a good pinch of pepper.
5. Put your pan back on a medium heat and pour in the eggs. Stir around gently with a spatula until you feel the egg start to set at the bottom, then straight away, turn the heat off so the frittata stays half cooked and quite runny.
6. Carefully place your frying pan under the broiler for 2 to 3 minutes, or until the frittata is golden and cooked through (check the center with a fork.) Put a plate over the pan and turn over quickly but carefully – the frittata should come right out. Serve hot or cold.