Place beef roast on a double thickness of aluminum foil. (Be sure the foil is big enough to wrap around the roast and seal.) Sprinkle the onion soup over meat and add (optional) garlic. Spread the mushrooms over soup.
Place on grill about 2 to 3 hours (the larger the roast and the higher the temperature the quicker it will be done - but for the tenderest roast, try to keep the temperature closer to 260°F and aim for the longest cooking time). Turn occasionally.
Keep the temperature in the grill between 250°F to 300°F. Roast will be done when the internal temperature reaches 150°F on a meat thermometer.
