Thaw fillets if frozen. Cut fish into 6 portions. Place in a single layer, skin side down, in a well-greased baking dish, 12x8x2 inches. Combine onion, orange and lemon juice, ornage rind, and salt. Pour over fish; cover and place in refrigerator to marinate 30 minutes. Sprinkle fish with nutmeg and pepper. Bake in a moderate oven, 350F, for 25 to 30 minutes or until fish flakes easily when tested with fork.