If you cannot obtain acorns, you might use chestnuts instead. Peel the acorns and cook for 15 minutes in water or white wine. When soft, drain and purée with the butter, cream, salt and pepper.
Cut four squares of filo pastry measuring 4 inches. Place the cheese and nuts in the center and close the pastry to form small bag shapes. Bake in the oven at 350 degrees F.
Trim the meat and cut into four pieces. Layer with anchovies. Melt the cheese in the game stock until smooth.
Fry the sirloin pieces until only just done, then remove from the pan. Add the cheese sauce to the pan juices and blend.
Serve the sirloins, coated with a little sauce, alongside the acorn purée and the nut parcels.