Preheat the oven to 160°C/fan 140°C/gas 3. Cut the casserole steak into 2cm cubes and toss in the flour. Heat the oil in a large casserole and fry the beef for a few minutes until browned all over – you may need to do this in two batches. Remove the beef from the casserole with a slotted spoon and set aside.
Add the onion, celery, garlic, carrot, parsnips, mushrooms and tomato purée to the pan and cook for 3-4 mins, then add the chopped tomatoes, thyme leaves and stock, mixing everything together well. Ladle into a 27cm x 12cm x 8cm deep ovenproof dish. Lay the slices of potato over the top of the beef, overlapping to create a layered effect. Cover with foil, place in the oven and cook for 1 hour.
After 1 hour, remove the foil and cook for a further 30 mins until the potato topping is golden and crusty and the filling is very tender. For adults, serve with cooked beetroot or a dollop of chutney.
Cook's tip: for a baby, place a mixture of the beef, vegetables and sauce in a blender, and whizz to a rough purée.