Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place fish on rack; grill, covered, 7 minutes on each side or until fish flakes easily when tested with a fork.
While fish cooks, combine yogurt and next 4 ingredients in a small bowl. Serve mustard-dillweed dressing with fish. Garnish with dillweed sprigs, if desired.