Melt butter, on stove top, in shallow 22cm flameproof dish; cook beef, in batches, stirring, until browned. Return beef to dish; sprinkle with flour, stir to combine. Gradually stir in sauce, mirin, sake and the water until smooth. Roast, covered, for 2 hours or until beef is tender. Season to taste.
Increase oven to 180°C (350°F).
Stir mushrooms into dish; season. Cover dish with pastry; brush with egg and sprinkle with seeds.
Bake pie for 15 minutes or until pastry is browned.