Combine oil and butter in a very large frypan over medium heat.   

Add mushrooms and saute for 8-10 mins until golden and reduced.  Add garlic and thyme and fry for a further minute. 

 Add beef stock, Worcestershire and onion powder, stirring well.  Increase heat to high and bring the liquid to a gently boil.  In a separate bowl, whisk cornflour with 125ml water until there are no lumps, then add to the mushroom sauce and whisk until combined.  Reduce heat to low, add cream and simmer until the gravy is a thick but pourable consistency.   

 Cook Boscastle Aussie Beef Party Pies according to packet instructions and serve with mushroom gravy.