Wash and peel potatoes, then thinly slice or cut into small cubes. Dice onion. Chop other optional vegetables.
Heat 2-3 tbsp olive oil in one pan, then add potatoes, onion and other vegetables if desired. Fry until potatoes are tender; drain excess oil and set aside. Beat eggs in a bowl, then add potato and onion mixture; mix well. Season with salt to taste.
Distribute remaining tbsp of olive oil among the two pans and warm both on separate burners over medium heat. Pour the egg mixture into the smaller diameter pan and cook until the eggs start to set. Drain oil from larger pan and lock it into place on top of the other pan, then gently flip the pan over. Cook on other side just until the eggs are set and the tortilla turns a light golden color. Remove from heat. Place serving dish over tortilla in pan, then flip over onto platter and serve.