Step 1: Season the Beef - Place the beef stew meat in a bowl and sprinkle it evenly with the kosher salt, black pepper, garlic powder, and onion powder. Toss everything together until the pieces are coated on all sides. If you notice any especially large chunks of beef, cut them down so the pieces are fairly similar in size. That helps everything become tender at roughly the same time.
Step 2: Brown the Beef - Heat the oil in a large skillet over medium high heat. Add the seasoned beef and let the pieces brown on the outside, working in batches if your skillet feels crowded. You are not trying to cook the beef all the way through here. You just want some good browned surfaces because those caramelized edges bring extra depth to the finished gravy. Once browned, transfer the beef to the bottom of a 6 quart slow cooker.
Step 3: Add the Vegetables - Scatter the diced onion, sliced carrots, sliced celery, and minced garlic over the browned beef. There is no need to saute the vegetables first. They have plenty of time to soften in the slow cooker, and as they cook, their juices mingle with the beef and gravy. This is one of those moments when you can appreciate having fewer pans to wash after dinner.
Step 4: Build 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. You do not need to fuss with making the mixture completely smooth at this point. As everything heats and cooks, the soup and broth come together into a rich gravy around the beef and vegetables.
Step 5: Slow Cook Until Tender - Cover the slow cooker and cook on LOW for 7 to 8 hours or on HIGH for 4 to 5 hours. The beef is ready for the next step when it is easily pierced with a fork and the vegetables are soft. I like using LOW when the schedule allows because that long, gentle cooking gives stew meat plenty of time to relax and become wonderfully tender.
Step 6: Add the Biscuit Dumplings - Open the refrigerated biscuits and cut each biscuit into 4 pieces. Arrange the pieces in an even layer over the beef mixture, keeping them mostly on top of the gravy. Press them down just enough for the bottoms to touch that savory liquid, but do not completely submerge them. Cover the slow cooker and cook on HIGH for 45 to 60 minutes. Resist the urge to keep checking them. The trapped steam is what helps the biscuit pieces puff into soft dumplings.
Step 7: Check the Dumplings and Add the Peas - Once the dumplings look puffed, check the center of one of the largest pieces. There should not be any wet or raw dough inside. Slow cookers can vary quite a bit, so this simple check matters more than watching the clock alone. When the dumplings are cooked through, gently stir the frozen peas into the beef and gravy, taking care not to tear up the dumplings.
Step 8: Thicken the Gravy - In a small bowl, whisk together the cornstarch and cold water until completely smooth. Gently stir that mixture into the gravy, moving around the dumplings as much as possible instead of stirring aggressively through them. Cover and cook on HIGH for another 5 to 10 minutes. The gravy should become glossy, thick, and rich enough to cling to the beef and vegetables.
Step 9: Finish and Serve - If any dumplings shifted while you thickened the gravy, gently arrange them back over the beef. Sprinkle the fresh parsley over the top if you are using it. Spoon the dinner into bowls while everything is good and hot, making sure every serving gets plenty of tender beef, vegetables, gravy, and at least a couple of those soft dumpling pieces.