Step 1: Season the Beef - Place the beef stew meat in a large bowl and sprinkle it with the kosher salt, black pepper, garlic powder, and onion powder. Toss everything gently until the pieces are evenly coated. It does not have to be absolutely exact. You just want seasoning on all sides so the beef starts building flavor before it ever reaches the slow cooker.
Step 2: Sear the Beef - Heat a large skillet over medium high heat and add the oil. Once the oil is hot, add the beef in a single layer and let it brown. You are not trying to cook the meat all the way through here. You simply want some good brown color on the outside because those browned edges add a deeper roasted flavor to the finished sauce. If your skillet feels crowded, brown the beef in batches so the meat sears instead of steaming.
Step 3: Build the Slow Cooker - Transfer the browned beef to the bottom of a 6 quart slow cooker. Scatter the diced onion and sliced mushrooms over the beef, then sprinkle the minced garlic across the top. Add the cream of mushroom soup, beef broth, Worcestershire sauce, and Dijon mustard. Stir everything together until the soup and broth are combined and the vegetables are nestled into the sauce.
Step 4: Slow Cook Until Tender - Cover the slow cooker and cook on LOW for 6 to 7 hours or on HIGH for 3 to 4 hours. The beef is ready when it is very tender and easily gives when you press it with a fork. I prefer the LOW setting when the day allows for it because the longer cooking time gives stew meat plenty of opportunity to soften, but the HIGH setting is useful when dinner sneaks up on you faster than expected.
Step 5: Add the Ravioli - About 30 minutes before you plan to serve dinner, give the beef and sauce a gentle stir. Add the refrigerated cheese ravioli and carefully fold them into the sauce, trying to get most of the pasta covered. Cover the slow cooker and cook on HIGH for 15 to 25 minutes. Start checking early because refrigerated ravioli can go from tender to overly soft faster than you might expect.
Step 6: Warm the Sour Cream - Place the sour cream in a medium bowl. Spoon about 1/2 cup of the warm stroganoff sauce into the sour cream and whisk until smooth. This small step is worth the extra bowl. It gradually warms the sour cream before it goes into the hotter slow cooker, which helps it blend smoothly and reduces the chance of the sauce separating.
Step 7: Finish the Creamy Sauce - Pour the warmed sour cream mixture into the slow cooker and gently stir until the sauce becomes creamy and evenly combined. At this point, take a look at the consistency. If the sauce already coats the ravioli exactly the way you like it, you can skip the cornstarch. If it seems a little thin, move on to the thickening step.
Step 8: Thicken and Serve - Whisk the cornstarch and cold water together in a small bowl until completely smooth. Gently stir the mixture into the slow cooker, cover, and cook on HIGH for another 5 to 10 minutes. The sauce should thicken enough to hug the ravioli and beef. Sprinkle with chopped parsley if you like, then spoon everything into bowls while it is warm and creamy.