Mushroom Slow Cooker Roast Beef

1 pound sliced fresh mushrooms

1 (4 pound) standing beef rib roast

1 (1.25 ounce) envelope onion soup mix

1 (12 fluid ounce) bottle beer

ground black pepper

Place the mushrooms in the bottom of a slow cooker; set the roast atop the mushrooms; sprinkle the onion soup mix over the beef and pour the beer over everything; season with black pepper. Set slow cooker to LOW; cook 9 to 10 hours until the meat is easily pulled apart with a fork.