Preheat the oven to 375 degrees F. In a saute pan, over medium heat, melt the butter. Add the onions. Season with salt and pepper. Saute for 2 to 3 minute or until wilted. Add the mushrooms and garlic. Season with salt and pepper. Saute for 4 to 6 minutes or until wilted. Remove from the heat and cool. Season both sides of the chicken breasts with salt and pepper. Place each chicken breast on one half of each pastry round. Spoon 1/2 cup of the mushroom mixture on top of each chicken breast. Fold the other half of the pastry round over the chicken, forming a half-moon shape. Fold the ends of the pastry in 1/2-inch and pinch the ends tightly to seal the ends completely. Place the chicken on a parchment-lined baking sheet. Brush each pastry with the egg wash and place in the oven. Bake for 18 to 20 minutes or until the pastry is golden. Remove from the oven. Spoon the sauce in the center of each plate. Place the pastry on top of the sauce and garnish with parsley.