I used prime sirloin steak, too, cutting it up and discarding every vestige of white fat. It went into a pan with oil to sear the surfaces. Then I put in the chopped field mushroom. After a few minutes it went into the pie base, and I put a couple of spoons of gravy in as well it before sealing on the top and brushing with egg and milk.
The potatoes were yesterday’s roast ones, oven heated under foil for 20 minutes. With the peas it made a good dish, and gravy was added just as I served it.