Beef and shiitake mushroom pie

Ingredients

• 30 gram butter
• 900 gram beef brisket, chopped coarsely
• 1/4 cup (35g) plain (all-purpose) flour
• 1/4 cup (60ml) light soy sauce
• 1/2 cup (125ml) mirin
• 1/2 cup (125ml) sake
• 1 cup (250ml) water
• 100 gram oyster mushrooms, chopped coarsely
• 100 gram fresh shiitake mushrooms, chopped coarsely
• 1 sheet puff pastry
• 1 egg, beaten lightly
• 1/2 teaspoon black sesame seeds
• 1/2 teaspoon white sesame seeds