Mix onion, cinnamon, soy sauce, sugar, salt, and pepper in small bowl for rub.
Trim fat from meat and score the top and bottom of the roast in a diamond pattern making cuts about 1/4 inch deep.
In a grill with a cover, arrange heat for indirect grilling. Heat should be medium-slow. Place meat on rack in a roasting pan on the grill. Cover and grill for 1 to 1 1/2 hours or until the meat thermometer registers 155°.
For the last 10 minutes of grilling, place apple rings on the grill rack until tender. Remove meat from grill. Cover with foil; let stand 15 minutes before carving.