Heat 2 tablespoons of olive oil in an 8- or 9-inch skillet over medium heat.
Add the potatoes and onions in alternating layers, seasoning each layer with salt.
Cook, turning occasionally, until the potatoes are tender but not browned, for about 20 minutes.
Drain in a colander and set aside.
Reserve 2 teaspoons of the oil and wipe the skillet clean.