Combine fresh dill, mustard, rind, juice, salt, pepper, and garlic in a bowl, stirring with a whisk. Add salmon pieces, tossing gently to coat. Thread salmon onto each of 4 (8-inch) skewers. Place salmon on grill rack coated with cooking spray, and grill 3 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness. Garnish with dill sprigs, if desired.