Heat 2 tablespoons oil in a large ovenproof skillet, preferably nonstick, over medium heat until it shimmers. Add the chouriço and cook until lightly browned, about 5 minutes. With a slotted spoon, transfer to a large bowl. Turn the heat to medium-low and toss the onions and potatoes into the skillet. Season with salt and pepper and cook, stirring often, until the onions are translucent and the potatoes are tender, 20 to 25 minutes.