Wash and peel the potatoes, cut 1 cm cubes. Clean the mushrooms and cut into slices. Heat the oil in a pan, fry the potato cubes in it, cook over a medium heat for 20 minutes. Add the mushrooms and ham, cook for another 5 minutes, leave to cool. Wash and quarter the cocktail tomatoes. Mix the tartar sauce with the milk, season with mustard and pepper. Mix all ingredients with the sauce and let it steep.