Using a slow cooker, ground beef is simmered low and slow with vegetables to give you something truly delicious. The potatoes are cooked with the meat, and then removed to be mashed for that classic, carby crust.
In a large skillet, heat oil over medium-high. Add beef and salt and cook while breaking meat apart with a wooden spoon until cooked through, about 5 minutes.
Add in celery, carrot and onion. Continue to cook until meat is browned and onions are translucent, 8 to 10 minutes. Stir in tomato paste, worcestershire sauce and thyme, followed by cornstarch. Transfer beef and vegetable mixture to a 4 L slow cooker. Place halved potatoes over mixture. Cover and cook on high until potatoes are soft, about 2 hours.
When potatoes are tender, reduce slow cooker heat to low.
Carefully remove potatoes from slow cooker and place in a medium bowl along with butter and salt, to taste. Mash until mixture is smooth and creamy. Spoon mashed potato mixture over meat and smooth top with the tines of a fork. Cover and continue to cook for another hour. Serve.