Heat grill. In small bowl, combine all mustard sauce ingredients; mix well. Cover; set aside.
With small metal spoon, scrape underside of mushroom caps to remove dark gills and stems. Cut each cap into 6 pieces.
Alternately thread mushroom pieces, onion, tomatoes and artichoke heart quarters onto two 12 to 14-inch metal skewers. Brush with artichoke marinade.
When ready to grill, place kabobs on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Cook 8 to 10 minutes or until mushrooms are tender and onions are crisp-tender, turning and basting occasionally with marinade. Serve kabobs with mustard sauce.