Gently saute diced onions and mushrooms in a little olive oil until softened. Season with salt and pepper.  Preheat oven to 425F.

While vegetables soften, grease a suitable baking dish with a little butter and  add one sheet of pie pastry.  Gently press to mould into the baking dish.  Don't worry about exactness or pastry slightly tearing, it will usually firm back together during cooking.

Once vegetables are softened, about 10-15 minutes, add ground beef and cook until mostly browned.

Add ketchup, Bisto gravy granules, soy and Worcestershire sauce and stir.  Cook for one minute then add water.  

Cook for 10-20 minutes, until a thick gravy consistency has formed.  The pie filling shouldn't be watery.

Pour pie filling into baking dish.  Top with second layer of pastry.

Using a fork crimp around the edges of the top of the baking dish to make a seal between the two pastry layers.  Using a knife work around the top edge of the dish to cut off any loose ends and make things tidy.  

Poke three or four holes into the pie for steam venting and put in the oven.  Check after 30 minutes for pastry cooking doneness.  I find the pastry I used takes a full hour to reach the deep golden brown finish I like.

The pie should be good to eat right away, but you might want to let it cook in the baking dish for 15 minutes first.  Once fully cooled I often keep the pie in the baking dish for upto two or three days to eat a slice cold, or microwaved/baked to temp again.