Preheat the oven to 170°C.
Trim hard fat from the beef and season both sides with salt and pepper.
Heat 15ml of oil in a large non-stick frying pan.
Brown the steaks for two minutes per side and transfer to a large casserole dish.
Fry the onion wedges in the remaining oil for five minutes until softened.
Add the garlic for one minute then transfer the mixture to the casserole dish.
Add the stock, tomato purée, thyme, and bay leaf to the dish.
Bring to a boil, cover, and bake in the oven for 75 to 90 minutes.
Whisk cornflour, mustard powder, and 15ml water then stir into the casserole.
Simmer the casserole on the hob for 3 minutes until the gravy thickens.
Serve the steak and gravy on warmed plates with chips.