Season the beef. Toss the beef stew meat with salt, black pepper, garlic powder, and onion powder until evenly coated. Brown the beef. Heat the oil in a large skillet over medium-high heat. Add the beef and brown the pieces on the outside. They do not need to cook all the way through. Add the ingredients to the slow cooker. Transfer the beef to a 6-quart slow cooker. Add the onion, carrots, celery, and minced garlic.
Make the gravy. Spoon the cream of mushroom soup over the beef and vegetables. Pour in the beef broth and Worcestershire sauce, then sprinkle the dried thyme over everything.
Slow cook the beef. Cover and cook on LOW for 7–8 hours or HIGH for 4–5 hours, until the beef is fork-tender and the vegetables are soft.
Prepare the biscuits. Cut each refrigerated biscuit into four pieces. Arrange the pieces in an even layer over the beef mixture, gently pressing them into the gravy without completely submerging them.
Cook the dumplings. Cover and cook on HIGH for 45–60 minutes, or until the dumplings are puffed and cooked through in the centers. Keep the lid closed while they cook.
Add the peas. Gently stir the frozen peas into the beef mixture, taking care not to break apart the dumplings.
Thicken the gravy. Whisk the cornstarch and cold water together in a small bowl until smooth. Gently stir the mixture into the gravy around the dumplings. Finish and serve. Cover and cook on HIGH for another 5–10 minutes, until the gravy thickens. Sprinkle with fresh parsley, if desired, and serve warm with plenty of beef, vegetables, gravy, and dumplings.