Peel the potatoes and cut them in ~1cm cubes

Peel and grate the onions, without discarding the liquid. This will give the omelet a smooth texture (as opposed to crunchy if the onions were chopped).

Pour 3 tbsp of olive oil in a big, tall, nonstick frying pan (better with vertical sides as it will be quite tall).

Throw in the potatoes and the onion and add the salt.

Fry the mix, medium-low heat, until the potatoes are tender and slightly golden brown (lower fire if they start getting more brown, the most important is the tenderness).

Whisk the eggs in a big bowl (optionally add the baking powder and the milk).

Add the fried potatoes and onion to the eggs in the big bowl and mix it well. Now it's the last chance to nail the salt levels so I recommend you to taste it and add more salt if needed. You can even add a touch of black pepper, if you want ;)

Clean the frying pan from the remnants of onion/potatoes with kitchen paper (it will get burnt if not done). Pour the remaining oil in it and wait for it to get warm. If your frying pan is nonstick you can reduce the amount of oil, but it will help on cooking the borders of the omelette better and faster (which is crucial to flip it without problems!).The ideal temperature of the oil is when splashing a drop off the mix it gets cooked almost instantly.

Pour the egg/potato/onion mixture to the frying pan, levelling it well. If the oil was at the right temperature the borders will start to cook right away.

When the egg is well curdled, lower the fire to the lowest strength and leave for around 4 minutes.

Now we want to cook the omelet evenly in all sides and especially in the borders. My mom's trick is to move the frying pan so that the stove heats just the top upper part, then the right part, then the lower and finally the left part, for about 2 minutes each (so if the stove was a clock, the frying pan should be at 12, 3, 6 and 9 for 2 minutes each). NOTE: if you're not using a gas stove you might need to repeat this step several times or even raise the heat to 3 for the borders.

Now for the trickiest part: to turn the omelet. This is how I do it: I take a big flat plate (or flat cover, or anything big, flat and strong enough to hold the weight of the omelet), I cover the frying pan with it, then I grab the frying pan handle with one hand and with the other one I press the plate downwards so I can easily turn the frying pan upside down, putting the omelet in the plate. Then I put the frying pan back in the fire and just slide the omelet back in. Phew!

Repeat the heating technique in step 12 in this side of the omelette, but start from the borders and do just 1-2 minutes in the center.

To see if the omelette is cooked, take a toothpick and insert it in the middle, down to the bottom. Then take it back slowly. If the toothpick is dry, the omelette is cooked. If not, repeat step 11 with 1 minute per side (you might need to flip it again as well, depends on how it's cooking).

After it's cooked, you can serve the omelette in round plate around the same size and remove the excess of oil from it with kitchen paper. I recommend it to be served at room temperature rather than hot, and for an even better touch leave it in the fridge for some hours and take it out 30 min before serving... I'm always told that it tastes even better the next day!

The traditional Spanish serving is by cutting the tortilla in cubes, like a grid, and put it on the table to share by using toothpicks. It's fun and way more social! Enjoy!

[Variant] Try using 1 big zucchini instead of the potatoes. Zucchini takes shorter to cook.