Coat slow-cooker bowl with nonstick cooking spray. Season beef with 1/2 tsp of the salt and 1/4 tsp of the pepper. Place in bottom of slow cooker.
Scatter onion, garlic, carrots, parsnips, turnips, potatoes and thyme over beef, in that order. Pour broth over top.
Cover and cook on HIGH for 5 hours or LOW for 7 hours. Drain liquid from slow cooker into a small saucepan and bring to a simmer. In a small bowl, combine cornstarch with 2 tbsp water and stir until combined. Slowly stir into saucepan and simmer 1 minute or until thickened. Season with remaining 1/2 tsp salt and 1/8 tsp pepper. Stir liquid back into slow cooker and serve.
