1 Optionally, skip the oven bag and cook the roast at 325 degrees Fahrenheit in a covered roasting pan for 1 hour and 10 minutes to 1 hour and 30 minutes or until the internal temperature reads at least 145 degrees.ips & Warnings. 1  Optionally, skip the oven bag and cook the roast at 325 degrees Fahrenheit in a covered roasting pan for 1 hour and 10 minutes to 1 hour and 30 minutes or until the internal temperature reads at least 145 degrees.