Stir in onion-mushroom mixture, salt & pepper. Cool to room temperature. Preheat oven to 180ºC & oil a cookie sheet. On a lightly floured board, roll one pastry sheet into a 28 cm X 36 cm rectangle. Cut into 8 cm circles & place 2 ts filling in the centre of each circle. Pull edges up around the filling, completely enclosing it. Pinch dough together to form a tight package. Turn packages over & place seam side down on a cookie sheet. Beat remaining egg substitute with water to make an "egg wash". Brush each knish with this wash & sprinkle with poppy seeds. Repeat with remaining sheets of dough. Bake until golden brown, about 30 minutes. Serve warm or at room temperature.