Potato, egg, and onion make tortilla española, a delicious, thick frittatalike cake that’s a Spanish classic. The best tortillas are homemade—restaurant versions tend to be pretty rubbery. Sometimes other ingredients like chorizo or greens are added. For this all-fish menu, I added shredded salt cod, but it is optional. Two cups of olive oil may seem like a lot, but it serves as a poaching liquid and can be saved and used again to make more tortillas or to cook vegetables.