Clean, wash and quarter the leek, then cut into fine strips. Sweat briefly so that it becomes a little soft (this works great in the microwave at 700 watts and 3 - 4 minutes). Let cool down.
Wash the apples, remove the core and cut the pulp into small cubes. Mix the leek and apple pieces in a salad bowl. Squeeze the lemon and pour the juice over the apple and leek mixture.
Mix the grated horseradish and sour cream with the milk until smooth, season with salt and freshly ground black pepper and season with agave syrup. Mix the sauce with the salad. Chill a little before serving.