Wash fruit well. Peel, core, and cut each apple into 8 to 10 slices, then cut each slice in half. Toss apples with lemon juice. Cut grapes in half and combine with apples. Stir in yogurt and nuts. Save out a few sprigs of mint for the garnish, then chop enough to make a scant 2 tablespoons, and stir it into the fruit mixture. Serve as a luncheon salad or a light dessert.