Slice the potatoes into thin rounds about 1/2 cm thick. Heat 2 tbsp olive oil in a medium non-stick frying pan (about 25cm across), then add the potatoes and season. Cover and cook gently for 10 minutes, stirring now and again, until tender.
Meanwhile, put the peppers into a blender, add the vinegar, 1 tbsp extra-virgin olive oil, the spices, garlic and some seasoning and whizz to a smooth sauce.
Heat the grill to high. Scatter the onions over the potatoes, then pour the eggs over and shake the pan to distribute them evenly. Leave the tortilla to cook for about 8 minutes, shaking the pan regularly, until the eggs are almost set. Flash the pan under the grill for 1-2 minutes to finish cooking then serve in wedges with salad and the sauce.