Saute onions and mushrooms in butter in a non-stick skillet over medium high heat until golden brown. Remove from heat and set aside.
Unfold pastry dough and cut into 2x2 inch squares.
Place 1 tsp of mushroom mixture in center of dough. Place 1 medallion on top of mix. Fold corners to the center. Flip over so seams are on the bottom and bake on non-stick cookie sheet until golden brown. If desired, you can brush them with egg whites before baking for a better browning job.