Place onion, potatoes, ¼ cup olive oil, and salt in two layers in a microwave-safe casserole that has a lid. Stir ingredients, cover, then microwave for 20 to 30 minutes on medium-high or until potatoes are very soft. You must stop the microwave about every 5 minutes to stir the potatoes so that the edges don’t get brown and crusty.
Meanwhile, beat the eggs in a bowl large enough to also accommodate the potatoes. Once the potatoes are done, pre-heat a 10’ non-stick skillet with 2 tbsp olive oil over medium heat.
Have ready a heat proof spatula or wooden spoon and a large plate. When the potatoes are cooked, add them to the eggs and gently stir. Then, pur the mixture into the skillet. The eggs will bubble up around the sides; so immediately push down the sides with a heat proof spatula. Then continue to gently shake the pan during cooking.
Cook on the first side for 3 to 4 minutes, or until it looks like the bottom is slightly browned. Use the large platter to invert the entire tortilla, and slide it back into the skillet to cook the other side. Continue cooking and shaking the pan for another 3 to 4 minutes. Finally, slide the cooked tortilla onto a serving plate and let it sit for at least 10 minutes before serving.
The tortillas can be served hot, at room temperature (more typical), or cold. Cut into wedges for large tapas or squares for a buffet.