Soak wood chips in water 30 minutes, and drain well.
Combine lemon juice, horseradish, and fillets in a large zip-top plastic bag. Seal and marinate in refrigerator 20 minutes, turning bag occasionally.
Prepare grill for indirect grilling, heating one side to low and leaving one side with no heat. Maintain temperature at 200° to 225°.
Place wood chips on hot coals. Place a disposable aluminum foil pan on unheated side of grill. Pour 2 cups water in pan. Coat grill rack with cooking spray; place on grill. Remove fillets from bag; discard marinade. Sprinkle fillets with 1/4 teaspoon salt and 1/4 teaspoon pepper. Place fillets on grill rack over foil pan on unheated side. Close lid; cook 30 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness.
Combine remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, vinegar, oil, and chili powder, stirring with a whisk. Combine greens, apples, and onion in a large bowl; drizzle with vinaigrette. Toss gently to coat. Remove skin from fillets; discard skin. Break fillets into pieces; serve over salad. Sprinkle with nuts.