Cut the top ¼ inch off the onion (leave the root end intacand peel the skin back to the root end. Wash the potatoes, but don’t bother peeling.
Using a mandolin set to medium (3mm), slice the onions and potatoes into 3mm thick rounds and toss with 1 tsp of the salt in a large bowl.
Heat the olive oil in a non-stick skillet on medium low for 1-2 minutes and immediately add the potato mix to the pan.
Allow to cook on medium low for 20-25 minutes until the potatoes and onions have given off most of their water and potatoes start to brown.
While the potatoes are cooking, mix in a medium bowl the eggs and remaining ½ tsp of salt until uniform in color.
Pour the egg mix on top of potatoes and cook for 5 minutes until the eggs set on the bottom (the top will still be uncooked). Finish under the broiler for 1-2 minutes until the top is just set.
This tortilla can be served hot, room temp or even chilled.