Peel onions, leaving root intact; cut each onion into 8 wedges.
Combine mustard and next 7 ingredients (mustard through salt) in a small bowl; reserve 1/4 cup mustard mixture.
Place onion, zucchini, and salmon in a large shallow dish. Drizzle with remaining mustard mixture; cover and marinate in refrigerator 1 hour.
Place a grill pan coated with cooking spray over medium heat until hot. Place zucchini on pan; grill 2 minutes on each side or until tender. Remove zucchini; keep warm. Place onions on pan; grill 3 minutes on each side or until tender. Remove onions; keep warm. Place salmon on pan; grill 5 minutes on each side or until fish flakes easily when tested with a fork. Drizzle salmon and vegetables with reserved mustard mixture.