2 lbs (900g) of beef tenderloin (trim fat)
coarse salt
freshly ground pepper
2 tbsp (30ml) unsalted butter
one small onion, finely chopped
flour for rolling pastry

2 cups (500 ml) mushrooms, finely chopped
1/4 cup (50 ml) dry sherry
1 lb (0.5 kg) frozen puff pastry, thawed
4 oz (115 g) chicken or goose liver pate at room temp
one large egg, lightly beaten
sea salt for sprinkling (optional)