Apples are peeled, cleaned of core and cut into thin slices, which immediately sprayed with lemon juice to not tarnish. Add pre-cut julienne "Emmental" grilled chopped walnuts. Mix everything well. Stir mayonnaise with mustard, a pinch of sugar and salt to taste. Apples and the feta cheese are covered with mayonnaise sauce. Salad garnish with parsley and serve chilled.