Combine first 10 ingredients in a large saucepan. Bring to a boil, and cook until reduced to 1 1/2 cups (about 15 minutes); remove from heat. Combine cornstarch and water in a small bowl; stir well with a whisk. Add to pan. Bring to a boil; cook 1 minute, stirring constantly. Remove from heat; stir in onions.
Place tuna on grill rack coated with cooking spray; grill 3 minutes on each side or until medium-rare or desired degree of doneness, basting frequently with glaze.