Place stew meat, mushrooms, and onions in a slow cooker. In a medium-sized bowl, whisk together broth, gravy mixes, Worcestershire sauce, garlic powder, onion powder, salt and pepper.
Pour over meat and vegetables. Cook on low for 7-8 hours or high for 4-5 hours. Just before serving, stir in parsley. Serve over mashed potatoes, rice, or egg noodles.
If you want the sauce thicker, make a slurry with 1 tablespoon cornstarch and 3 tablespoons water. Stir into meat mixture and leave the lid off of the slow cooker for 15 minutes before serving (but leave heat on).