A good steak & kidney pie

INGREDIENTS

• 250g plain flour , plus extra for rolling
• 140g cold unsalted butter , roughly cubed
• 1 large egg yolk
• 1 small egg whisked with 1 tbsp milk, for the egg wash
• 1 ox kidney , about 400g/14oz, get it fresh from your butcher. Pig's and lamb's kidneys aren't suitable as they only need short cooking
• 1kg trimmed British top rump beef
• 250g flat mushrooms , unpeeled but wiped with a damp cloth
• 3 tbsp groundnut or vegetable oil
• 100g unsalted butter
• 1 large onion , peeled and thickly chopped
• 50-85g plain flour , depending on how thick you like your gravy
• 600ml off-the-boil water
• 1 tbsp Marigold Swiss vegetable bouillon powder
• 1 large bay leaf