Method

1. Using a sharp knife, cut the onion in half and peel off the skin. Then put each half on a chopping board and cut into slices, keeping the root intact. Then make 2 horizontal cuts one above the other and finally, chop down across the width of the onion. 
2. Peel the potato and cut and remove the seeds of the red pepper and cut both into dice.
3. Heat a large non-stick frying pan over a medium heat. Add the butter and then tip in the onion and cook for 2-3 minutes, until well softened. Add the potato and red pepper and continue to cook for 6-8 minutes until the potato is just tender when pierced with a sharp knife. Make sure to stir regularly to ensure that it doesn't catch at the bottom of the pan.
4. Break the eggs into a bowl, add the milk and season with the salt and pepper. Beat with a fork until evenly combined, then crumble in the feta cheese and teat in the basil. Set aside.
5. Once the potato is cooked, add the spinach and salad leaves to the onion mixture in fistfuls and stir until wilted. 
6. Spread out the vegetables evenly in the pan and add the egg mixture. Reduce the heat to low and cook gently for 10 minutes until the frittata is softly set and golden underneath. 
7. Meanwhile, preheat the grill to medium and then put the frittata under for a minute or two until the top is set and golden. Cut into slices and serve on plates with the salad leaves.