Wash the potatoes and cook covered for about 20 minutes. Then quench, peel and let cool off.
Clean, wash and cut the peppers into strips. Sort the spinach, wash, shake dry and chop coarsely. Peel onion and garlic. Cut onion into strips, chop garlic. Cut potatoes into slices.
Heat 3 tablespoons of oil in an ovenproof pan (28 cm Ø; with lid). Fry the potatoes for 4-5 minutes while turning. Season with salt, pepper and paprika. Add paprika, onion and garlic and fry for about 4 minutes.
Dice the feta, fold into the potatoes with spinach.
Whisk eggs, milk, salt and pepper and pour over the mixture. Cover and allow to set at low heat for approx. 10 minutes. Continue cooking in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for 20-30 minutes.
Wash the parsley, shake dry. Chop coarsely with the capers. Mix with 2-3 tablespoons of oil, season with salt and pepper. Remove the tortilla from the oven and let it rest in the pan for about 5 minutes. Turn out, sprinkle with parsley-capers mixture.