Slice the stem of the artichokes and remove the outer leaves. Rub each artichoke heart with a lemon and put all of them in a saucepan. Boil them in salted water until they soften. In a bowl mix the cod, the potato mash, the garlic, two spoons of oil, salt and pepper. Mix well and try to make a soft and creamy mixture. Stuff the artichoke hearts with the cod mixture and serve.