In a saucepan, heat 2 tablespoons of the olive oil. Add the onion and cook over low heat until softened but not browned, about 5 minutes. Add the yellow peppers, potato, water and a large pinch of salt. Cover and cook until the peppers and potatoes are tender, about 12 minutes. Transfer the vegetables and their liquid to a food processor and process until roughly pureed. Scrape the sauce into a bowl, add the lemon juice and season with salt and pepper.