HEAT oven to 375ºF.
MINCE enough onion to measure 1/4 cup; set aside. Cut remaining onion into wedges. Combine potatoes, carrots and onion wedges in large bowl. Add oil; toss to coat. Spread onto bottom of 15x10x1-inch pan. Bake 15 min.
MIX turkey, minced onions, cracker crumbs and egg white in same bowl. Remove vegetables from oven; stir, then push to edges of pan. Shape turkey mixture into 6x3-inch oval loaf; place in center of pan. Brush with barbecue sauce.
BAKE 15 to 20 min. or until meatloaf is done (165ºF) and vegetables are tender. Sprinkle cheese over vegetables.