Season the cubed steak with salt and pepper.
Heat the vegetable oil in a large skillet over medium-high heat and sear the beef until browned on all sides.
Transfer the browned beef to the slow cooker basin.
Add the onion, carrots, celery, and mushrooms to the slow cooker.
In a separate jug, whisk together the beef stock, tomato puree, Worcestershire sauce, and dried thyme, then pour the mixture over the meat and vegetables.
Cover and cook on Low for 7 to 8 hours or High for 4 hours until the meat is tender.
Thirty minutes before the end of the cooking time, stir the cornflour slurry into the casserole to thicken the sauce.
Adjust seasoning to taste and serve hot.