Peel and boil the potatoes, just until tender. Avoid overcooking or the potatoes will not slice nicely. Slice thinly. Using a mandolin slicer produces uniform slices, but it's not essential. A sharp knife will work too.
Heat half of the oil in an ovenproof skillet over medium heat and sauté the onions, just until translucent but not browned.
Whisk eggs in a large bowl. Add the sliced, cooked potatoes, salt and onions.
Add the remaining oil to the frying pan and when hot, pour in the egg-potato mixture. Let it cook for a minute or two, just until the edges are set.
Transfer the skillet to the oven and bake at 350 F. for 20 minutes or until the egg has set.
Invert a serving dish over the frying pan and flip the frying pan over. The tortilla will release easily.
Allow to cool. Slice into wedges or small squares and serve at room temperature.