Season beef chunks generously with salt and pepper.
Heat vegetable oil in a skillet over medium-high heat.
Brown beef in batches until a deep crust forms, then transfer to the slow cooker.
Place root vegetables at the bottom, then add beef on top.
In the same skillet, sauté garlic briefly, then stir in tomato paste until slightly darkened.
Pour the mixture into the slow cooker and add thyme and beef broth.
Cover and cook on LOW for 7–8 hours or HIGH for 4–5 hours until beef is fork-tender.
If thicker stew is desired, stir in cornstarch slurry and cook on HIGH for 15–30 minutes until thickened.
Let stew rest 10–15 minutes before serving for best flavor and texture.
Taste and adjust seasoning, then serve hot.