Cook celeriac in saucepan of boiling water until cooked through, about 8 minutes. Drain. Cut into thin strips, 2 x 1/4 inches.
Mix the mayonnaise, egg, parsley, capers, anchovy paste, mustard and tarragon in bowl; add celeriac.
Mix sour cream, horseradish, lemon juice, salt and pepper in small bowl.
Place 1 fillet on each of 8 chilled plates. Garnish with celeriac salad and horseradish cream. Makes 8 servings.