In a separate bowl, whisk together onion soup mix, cream of mushroom soup, water, garlic, Worcestershire sauce and parsley flake. Pour over roast.
Add chopped onions and sliced mushrooms over top of roast.
Cook on high for 6 hours or until beef is very tender. Use tongs to remove beef to a cutting board. Use two forks to shred beef into bite size pieces, removing any large fat pieces.