Cut beef, leeks and carrots into 3cm pieces. Heat an ovenproof casserole dish over high heat. Add 1 tablespoon oil and half the beef, cook 4 minutes until evenly browned, remove to a plate. Repeat with remaining beef. Add remaining oil, leeks, carrots and mushrooms. Cook, stirring often, for 5 minutes. Sprinkle over the paprika, season and stir to coat. Return beef to the dish.
Add tomatoes and stock. Bring to the boil. Press a piece of baking paper onto the surface of casserole. Cover with a lid, reduce heat to very low and simmer 3-4 hours until beef is tender. Remove baking paper and discard.
Preheat oven to 240°C or 220°C fan forced. Combine flour, polenta, buttermilk, cheese and baking powder, mix until well combined. Drop spoonfuls of the cheese mixture over the casserole to cover. Bake, uncovered, 15-20 minutes until golden and cooked through. Serve.