Separate crescent dough into eight triangles and place in an ungreased 9 inch pie plate with the points toward the center. Press onto the bottom and up the sides to form a crust; seal the perforations. Spread with the mustard and set aside.
In a large skillet, saute the asparagus, onion and mushrooms in butter until the asparagus is crisp and tender.
In a large bowl combine the remaining ingredients and stir in the asparagus mixture. Pour into crust. Bake at 375° for 25 to 30 minutes or until a knife inserted near the edge comes out clean. Let it rest for 5 minutes beofre cutting.
Special Note: I always steam my asparagus when I make it so I did so for this recipe. I just added it to the mushrooms and onion once they were tender. I've also made my own crust instead of using the crescents both are very good.