In about a 10 or 12 inch non-stick skillet, add olive oil just to cover the bottom. Not too deep. Turn heat to medium / medium low.
Add potatoes to skillet. Cook slowly, turning frequently. Adjust heat so that potatoes begin to change color to somewhat translucent. The goal is to "poach" the potatoes until they become very soft and start to fall apart. Avoid browning / crisping potatoes.
After about 10 minutes (potatoes about halfway done), add sliced onion. Sprinkle generously with salt (any type you prefer).
When potatoes and onions are well cooked, use a wooden spoon or something that won't scratch the skillet, and break the potatoes and onions down into very small pieces. Taste and adjust salt.
Remove pan from heat. Drain excess oil from potato mixture. With wooden spoon or similar, pull potato mixture to one side of pan, and tip slightly to allow oil to drain to opposite side. Spoon oil out of pan into a small jar (keep the oil - it's great too cook with!). Press the potatoes with back of spoon to remove as much oil as possible. If necessary, pat with paper towels to remove more oil.
To prepared beaten egg in large bowl, add cooked potato mixture. Mix well. See video - proportion of egg to potato mixture is important!
Put skillet on burner on medium / medium-low heat. Bottom of pan should have a bit of leftover oil. Pour potato / egg / onion mixture into skillet.Using wooden spoon or non-stick spatula, gently tuck under the edges as the egg begins to cook. This shapes the tortilla edges. No need to cover. Allow to cook undisturbed for a few minutes until it starts to set up. Gently pierce the top of the tortilla all over with a fork - take care not to scratch the bottom of the pan.
After about 10 minutes, gently shake the pan. Egg should be set up, and tortilla should easily slide in pan. If not, carefully lift edges and run a non-stick spatula underneath to loosen any stuck areas.
When tortilla is pretty well set up (top will still be a little bit wet), if you're very brave, flip the tortilla onto a flat pan lid and slide the wet side of the tortilla back into the pan. See video. I prefer the safer route: Put a large, flat plate on top of the skillet. Hold the plate and the hot skillet together VERY TIGHTLY with 2 clean kitchen towels (CAREFUL not to burn yourself!). Go to the sink, and clamping the 2 parts together, quickly flip the tortilla out on the plate, so wet side is on top. Put skillet back on burner, gently slide tortilla into skillet. Continue to cook for just a few minutes until tortilla is as done as you like it. We prefer it moist on the inside. See photos. Tortilla can be served warm or at room temperature. We love it with good crusty bread drizzled with olive oil, and served with Espencat (roasted Spanish vegetables). The PERFECT Spanish comfort food!