Place sliced onions in your slow cooker, followed by water, the stew beef pieces, salt, pepper and Worcestershire sauce. Cook on high for 5-6 hours or until the beef is pull-apart tender.
Drain the cooking liquid reserving it for later use.
In a large skillet over medium high heat, add 2-3 tbsp of cooking liquid and sliced mushrooms. Cooked for 3-4 minutes. Add back the beef and onion mixture.
Add the 1 1/2 c of the reserved cooking liquid, bringing it to a boil. Add the heavy cream, mustard and sour cream. Stir and cook until well combined. Taste for seasoning, adding salt and pepper as needed. It should reduce slightly.
Serve over cooked egg noodles and garnish with parsley.