In a 9-inch cast iron skillet (or other nonstick pan), heat the olive oil on medium high heat. Add the potato slices in a single layer, then add a layer of onion slices, and repeat, salting between layers.
Once the potatoes are “boiling” in the oil, reduce the heat to medium and cook slowly. Stir the mixture periodically, breaking up the layers and exposing the vegetables to the oil. Allow to cook until potatoes are well softened but not brown.
Place a colander over a saucepan, and drain the potatoes. Reserve the oil.
In a large bowl, beat the eggs and then add the potato mixture to the bowl. Stir to coat the potato mixture completely.
Heat oven to broil.
Clean the skillet to remove any bits of potato stuck to the bottom of the pan. Once clean and dry, put 2 tablespoons of the reserved oil into the pan and heat on medium high heat, until oil is very hot.
Pour the potato egg mixture into the pan and arrange so bottom of the skillet is completely covered. Place pieces of pulpo into the mixture, pressing them in lightly so they are embedded in the mixture. Cook the tortilla until bottom is completely set and light to medium brown.
Place skillet in the oven on the top rack (about 6-8 inches from the heating element) and cook until top is set (about 2 minutes). Remove from oven and let sit for 15 minutes.
When ready to serve, invert skillet over a large plate so the tortilla comes out cleanly. Cut into slices and garnish with pimentón, serve with Matiz All I Oli.