Place a 3 to 4 pound beef pot roast in center of double thickness of heavy-duty foil. Sprinkle all sides of meat with 1 envelope dry onion soup mix. Spoon one 10 1/2 ounce can condensed cream of mushroom soup atop. Seal meat in foil. Place in 13 x 9 inch pan. Bake at 350 degrees for 2 1/2 to 3 hours. Remove meat to platter. Skim off excess fat from meat juices. Serve juices with meat or thicken for gravy. For a more onion-flavored roast, omit the mushroom soup.