Wash the potatoes, if they are bigger than 4cm, cut them in half. Boil in slightly salted water and cook until tender, about 10 minutes.
Meanwhile, cut the onion in half and slice thinly.
Heat the oil in a frying pan, sauté the onions gently for about 5 minutes. Finely chop the garlic, add to the onions with the capers. Stir and fry for a minute .
Drain the potatoes, add to the onion. Crush them gently with a fork.
Add the spinach and stir for a couple of minutes until a spinach wilted. Season to taste and squeeze some lemon juice over.