Preheat the oven to 350 degrees F.
In a large cast-iron skillet, heat the remaining 2 tablespoons oil over medium heat. Pour the egg, potato and onion mixture into the pan, shaking to evenly distribute. Carefully stir until level and then cook just until it begins to set, about 5 minutes. Transfer to the preheated oven and cook until set, 20 to 25 minutes.
Here comes the fun part. Remove from the oven, and place a large cutting board or plate over the skillet. Carefully invert the skillet to transfer the tortilla onto the board. Gently slide the tortilla back into the pan and return to the oven for another 10 minutes. Remove from the oven and immediately remove from the pan. Cool the tortilla 20 minutes before slicing and serving.