Pat the beef dry and season with salt, pepper, and smoked paprika (if using).
Optional: Sear the beef in a hot skillet with a little oil for 2–3 minutes per side, in batches, until browned. Transfer to the slow cooker.
Add the onion and potatoes to the slow cooker. Sprinkle the garlic and thyme over the top.
Whisk together the melted butter, beef broth, and Worcestershire sauce. Pour over the beef and potatoes.
Cover and cook on LOW for 7–8 hours (or HIGH for 4–5 hours), until the beef is fork-tender and potatoes are cooked through.
Taste and adjust salt and pepper. Stir in parsley (optional).
Optional thick sauce: Remove beef and potatoes to a serving dish. Transfer cooking liquid to a saucepan. Whisk cornstarch with cold water, then whisk into the hot liquid. Simmer 3–5 minutes until thickened. Pour over the beef and potatoes.