Heat the olive oil in a pan. Cut the potatoes into thin slices. Put them in the pan, along with the chopped onions and salt.
Put a lid on and saute for 20 min. Beat the eggs in a deep bowl. Take the tortilla out of the pan and use a slotted spoon to put the potatoes in with the eggs.
Season with salt and black pepper. Pour a little olive oil in a pan and heat it well.
Pour in the entire mixture and cook 2-3 min. Cover the pan with a plate and flip the tortilla over, put it back in the pan and cook another 5 min. Serve with cherry tomatoes.