Begin by preparing the flavorful filling. Cut the stewing steak into small, bite-sized pieces and slice the mushrooms. In a large pot, heat a combination of butter and oil, then sauté diced onions and minced garlic until they turn translucent. Add the beef chunks and cook until they achieve a golden brown color. Introduce the sliced mushrooms and continue simmering until they soften. Season the mixture with dried thyme, salt, pepper, tomato puree, and Worcestershire sauce. Gradually pour in the beef stock, ensuring all ingredients are adequately covered. Bring the mixture to a boil, then reduce the heat, cover the pot, and let it simmer for 1.5 to 2 hours until the beef becomes exceptionally tender. If the gravy requires thickening, have a cornstarch slurry (1 tbsp cornstarch mixed with 2 tbsp water) on standby, adding it and stirring until the desired thickness is achieved.
Moving on to the suet crust, combine self-raising flour, cornflour, shredded suet, salt, and optional dried thyme in a large bowl. Gradually add milk to the dry ingredients, stirring until a dough forms. Be cautious not to overmix. On a floured surface, shape the dough into a ball and roll it out to the desired size.
Now, let's assemble and bake this delightful pie. Pour the prepared beef filling into a baking dish. Cover the filling with the rolled-out suet crust, ensuring the edges are well-sealed. To allow steam to escape during baking, make a few small slits in the crust. Place the baking dish in a preheated oven set at 170 degrees Celsius and bake for approximately 20 minutes or until the crust achieves a golden brown color.