Peel the mango and cut into bite-sized pieces, place in a salad bowl with the lamb`s lettuce and coarsely chopped walnuts and stir well. Mix yoghurt, mustard, lemon juice and sugar into a dressing, season with salt and pepper to taste. If the dressing is too thick, add a little cold water and stir well. Just before serving, add the dressing to the salad bowl and stir everything well.