Melt the butter in a large frying pan over medium heat.
Add the onion and mushrooms, saute until the liquid evaporates.
Add the flour, whisking until smooth.
Cook for 1 minute, stirring constantly.
Add the lemon juice, stirring constantly.
Add the cream and cook over medium heat, stirring constantly until the mixture is thickened and bubbly.
Stir in the salt and pepper, to taste.
Remove from heat.
Roll out the pastry on a floured surface to 1/8 inch thick.
Cut into 2 1/2 inch squares.
Place a heaping teaspoon of the filling in the center of each square.
Grasp all 4 corners and pinch the four seams to form a bundle, drop into the cup of a mini muffin pan (Do not force them down).
Repeat with the remaining pastry and filling.
Cover and chill.
Preheat the oven to 400 degrees F.
Bake for 20-25 minutes or until sides and bottoms are browned.
