At least 1 hour before smoking, soak wood chips in the 3 cups apple juice. Drain before using.
Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Lightly grease the rack of a gas grill. Preheat grill. Adjust for indirect cooking over medium heat. Add soaked wood chips according to manufacturer's directions. Or, wrap in foil and add to grill. Cover and heat about 10 minutes or until chips begin to smoke.
For sauce, in a small skillet melt butter over medium heat. Add hazelnuts and cook, stirring occasionally, until nuts are toasted and butter is browned but not burned. Remove from heat. Stir in the 1 tablespoon apple juice. Serve immediately over grilled fish. If desired, garnish with parsley. Makes 4 servings.
To grill fish on a charcoal grill, arrange preheated coals around a drip pan; add soaked wood chips to coals. Test for medium heat above drip pan. Place fish on grill rack over drip pan. Cover grill; cook and serve as above.