Steps
- Pulse 250g of all-purpose flour, 1 pinch of fine sea salt, and 140g of cold cubed unsalted butter to coarse crumbs. Add 1 large egg yolk beaten with 3 tablespoons of water, pulse to bind, wrap, and chill for 1 hour.
- Trim the white core from 400g of ox kidney and cut into small lobes. Cut 1kg of trimmed top sirloin beef into bite-size pieces and 250g of portobello mushrooms into chunks.
- Heat one-third of 3 tablespoons of vegetable oil in a skillet over medium-high heat. Brown the kidney and drain in a colander.
- Wipe the skillet, add another third of the oil and a quarter of 100g of unsalted butter, and soften 1 large coarsely chopped onion over medium-low heat for 10 minutes. Transfer to a casserole.
- Preheat the oven to 325°F / 160°C. Toss the beef in 85g of seasoned all-purpose flour; reserve excess. Brown in the remaining oil and butter over medium-high heat and add to the casserole.
- Fry the mushrooms for 2 minutes. Add to the casserole with the kidney, reserved flour, 600ml of hot water, 1 tablespoon of vegetable bouillon powder, and 1 large bay leaf; stir, cover, and bake for 75-90 minutes, until tender.
- Cool, refrigerate overnight, and remove the solidified fat. Roll the pastry ⅛in / 3mm thick; cut a lid for a 12 × 9in / 30 × 23cm dish, 2½in / 6.5cm deep, leaving an extra ½in / 1cm around it.
- Cut 2½in / 6cm strips from the trimmings and attach to the buttered rim, extending partway down the inside. Place a buttered pie funnel in the center, add the cold filling, and moisten the pastry rim.
- Lay the lid over the filling, seal, and cut four vents. Preheat the oven to 400°F / 200°C; beat 1 small egg with 1 tablespoon of milk and brush over the lid.
- Bake for 20 minutes, lower the oven to 375°F / 190°C, and bake for 15-25 minutes, until golden. Rest for 10 minutes.