In a casserole dish, over high heat, drizzle some oil and add the short ribs. Season well with salt and pepper. Sear the short ribs 4-5 minutes on both sides or until well coloured.
Remove the beef pieces from the pan onto a side dish.
Add the onions, leeks, mushrooms and garlic to the casserole dish and sear, over medium heat, the vegetables.
Next, pop in the sprigs of thyme and add the Guinness and beef stock and tomato puree.
Add the beef pieces back into the casserole and season well with salt and pepper.
Bring the mixture to the boil. Then cover with a lid, reduce the heat to a low / simmering heat and simmer for 90 -120 minutes or so until the meat is soft and tender enough to remove the bone from the flesh and the flesh is easy to break. Alternatively, place in the oven at 150˚C and slowly cook for 2 hours tightly fitted with a lid.
Remove from the heat and serve with some roasted baby potatoes.