Pat the beef dry with paper towels and season with salt, pepper, garlic powder, and onion powder.
Heat olive oil in a large skillet over medium-high heat and sear the beef for 2–3 minutes until browned on all sides (optional but recommended).
Transfer the beef to the slow cooker.
Add the diced onion, garlic, sliced mushrooms, beef broth, brown gravy mix, onion soup mix, Worcestershire sauce, tomato paste, and thyme.
Stir everything together until well combined.
Cover and cook on LOW for 7–8 hours or HIGH for 4–5 hours, until the beef is fork-tender.
In a small bowl, whisk together the cornstarch and cold water.
Stir the slurry into the slow cooker.
Cover and cook on HIGH for 15–20 minutes, until the gravy thickens.
Garnish with fresh parsley.
Serve hot over mashed potatoes, buttered egg noodles, rice, or biscuits.