Preheat the oven to 190°C (375°F/Gas 5). Lightly grease two baking trays with melted butter or oil. Heat the butter in a large frying pan. Add the garlic and mushrooms and cook over medium heat, stirring occasionally, until the mushrooms are tender and the liquid has evaporated. Remove from the heat and cool. Stir in the capsicum.
Combine the sour cream, mustard and cheese. Cut out twelve circles with a 14cm (5½inch) diameter from the pastry. Spread the cream mixture over six of the circles, leaving a 1cm (½inch) border. Top each with some of the mushroom mixture. Sprinkle with 2teaspoons of the extra cheese.
Brush the outer edges with the beaten egg then place the reserved pastry rounds on top of the filling, sealing the edges with a fork. Brush the tops of the pastry with egg and sprinkle the remaining cheese over the pastry. Place the pies on the prepared trays and bake for 20 minutes, or until lightly browned and puffed.