This dish is based on tortilla Espanola. In Spain, it is traditionally accompanied by garlic mayonnaise but it is also delicious served on its own. You can use leftover boiled or baked potatoes in place of the raw ones. Simply slice them, and add to the onion mixture at the end of step 2. It's okay if the potatoes do not cook evenly. Some will be browned; some will only be translucent.