For the suet pastry, sift the flour and salt into a food processor, then add the suet and pulse until the mixture resembles breadcrumbs. With the mixer running, add the cold water, repeating until the mixture comes together to form a sticky dough.
For the pie mix, dust the diced beef in flour and season. Add some oil to a large, heavy-based pan and heat until very hot. Add the beef and fry until golden brown all over. Once the beef is browned, remove from the pan and set aside in a large bowl.
Add the onion to the same pan, fry until lightly golden brown in colour and add the thyme and garlic. Cook for a further 2-3 mins, then add the beef back into the pan. Add the stock, tomato puree and Worcestershire sauce, Leave to simmer for 2 hours then take off the heat and allow to cool slightly.
Remove the suet pastry from the fridge 1 hour before you need it.
Brush a 1l pudding bowl with softened butter. Roll out 3/4 of the suet pastry and use to carefully line the prepared bowl, ensuring there are no cracks or holes in the pastry.
Mix the oysters with the cooled beef mixture and add to the pudding bowl.
Roll the remaining 1/4 of pastry to a piece large enough to form a lid. Brush with a little water, then place onto the pudding, pinching and folding the edges to seal. Trim the edges of the pastry.
Cut out a large disc of parchment paper and the same sized disc of tin foil. Place the paper on top of the foil, and fold to form a large pleat. Place over the pudding and tie in place with string.
Place the pudding into a steamer and steam for 2 1/2-3 hours, topping up with boiling water as necessary. Carefully remove and turn out onto a large, deep serving dish - big enough to catch all of the gravy. Serve immediately.