Wash and peel potatoes, then thinly slice or cut into small cubes. Bring a large pot of well-salted water to a boil. Add potatoes and boil for about 5 minutes, until potatoes are almost tender. Drain potatoes well. Pat the potatoes dry and put them in a large bowl, sprinkle with salt and mix well. Dice onion. Chop other optional vegetables.
Heat 3 tbsp olive oil over medium-low heat in one pan, then add onion. Fry slowly, stirring occasionally until tender. When onions are transparent, add potatoes and optional vegetables to the pan. Allow to cook until potatoes are tender, stirring occasionally. Using a strainer, drain excess oil from potato mixture and set aside. Beat eggs vigorously in a bowl with a large pinch of salt, add potato and onion mixture; mix well. Let the egg, potato and onion mixture rest for at least 15 minutes before cooking.
Distribute remaining tbsp of olive oil among the two tortilla pans and warm both on separate burners over medium heat. Pour the egg mixture into the smaller diameter pan and cook over medium-low heat until the eggs start to set. (Cooking the tortilla on too high a heat will make it rubbery.) Run a heatproof spatula along the edges to loosen and make it easier to flip. Drain oil from larger pan and lock it into place on top of the smaller pan. Working over the sink, gently, but quickly, 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. Invert the serving dish onto the tortilla in pan, then flip over onto platter.
Cook’s note: If using a 9-inch skillet, add ½ tbsp olive oil and follow the instructions for tortilla pan in step 3, except invert a plate over skillet and flip omelette onto the plate. Then slide omelette back into the same pan to continue cooking.
Serve room temperature as a tapa. Or serve warm with seasonal fruit, a crisp green salad and crusty rolls for a lovely brunch!