To make tortilla, heat 60ml (1/4 cup) olive oil in a 25cm non-stick frying pan over medium heat, add onion and sauté for 3-5 minutes or until softened. Using a slotted spoon transfer to a small bowl, leaving any residual oil in the frying pan.
Meanwhile, lightly beat the eggs in a large bowl and season generously with sea salt and freshly ground black pepper. Add potatoes, onions and paprika and combine well. Let mixture marinate for a couple of hours in the refrigerator, preferably overnight. Remove tortilla mixture from refrigerator 30 minutes before planning to cook it.
To make tortilla, heat the frying pan again, adding remaining 1 tbsp olive oil, over medium heat. When oil begins to smoke slightly, add tortilla mixture to pan, shaking pan vigorously so that egg mixture amalgamates in bottom of pan. Reduce heat and cook for 4-5 minutes or until mixture all but sets. Place a plate over the frying pan and carefully invert the frying pan so that tortilla is on plate uncooked side down. Return frying pan to heat, carefully slide tortilla back in and cook for a further 4-5 minutes until tortilla is firm to the touch.
Transfer to a serving plate and allow to cool before serving. Cut into wedges and serve with Piquillo peppers and alioli if desired.