Boil the potatoes in their skins, peel them and cut them into cubes. Peel the carrots, quarter them lengthways and cut into slices, cook with the peas.
Peel the apple, dice finely, drizzle with lemon juice. Mix the mayonnaise, yoghurt, mustard and sugar into a cream, season with salt and pepper.
Mix the vegetables, apple and cream thoroughly. Put it cold and serve well chilled (it rarely works with us, the family pounces on it when it is still lukewarm )