In a small saucepan, saute the mushrooms, green pepper and onion in butter until tender. Add the tomato paste, water, oregano and garlic powder. Reduce heat to medium-low. Stir in cheese until melted. Remove from the heat.
Cut 3-1/2-in. circles from pie pastry. Place 1 teaspoon filling in the center of each circle. Brush edges of dough with water. Fold each circle in half; seal edges with a fork. Brush the tops with beaten egg. Place the turnovers on a greased baking sheet.
Bake at 425° for 12-14 minutes or until golden brown.
Editor's Note: Turnovers may be frozen, unbaked, for up to 2 months. Before serving, bake at 425° for 16-18 minutes or until golden brown.