Preheat oven to fan 180C/ conventional 200C/gas 6. Heat 2 tbsp of olive oil in a pan. Tip in onion and potatoes and fry gently for 20 minutes until the potatoes are tender, turning frequently, adding garlic and peppers for the last 5 minutes. Tip into a large bowl, season and cool for 5 minutes.
Stir in the eggs, chili flakes and parsley. Leave to sit for 5 minutes, then put a nonstick 20cm square tin in the oven to heat up. After 5 minutes, remove tin and brush with oil. pour in the egg mixture and return to oven.
Cook for 15-20 minutes. (Check it's ready by pressing the top lightly. If it is still runny, return to the oven for a couple of minutes.) Once cooked, remove and leave to cool for 5 minutes, then turn out on to a board. Cut into squares and serve.
To make ahead do as above then just before your guests arrive, cut the tortilla into squares, put on a baking sheet, cover with foil and warm through at fan 160C/ conventional 180C/gas 4 for 10-15 minutes, or serve cold.