Season roast with salt and pepper; place over indirect heat on grill; stir together cranberry sauce and onion soup mix and heat, covered, in microwave until hot (about one minute).
Baste roast with cranberry mixture every 10 minutes until roast is done about 40 minutes (20 minutes per pound);until internal temperature on a thermometer reads 145 degrees Fahrenheit, followed by a 3-minute rest time.
Remove roast from heat; let rest about 10 minutes before slicing to serve.
Heat any leftover basting mixture to boiling, stir and boil for 5-10 minutes and serve alongside roast.
