Preheat the oven to 400 degrees F (200 degrees C).
Melt the butter in a large skillet over medium heat. Add the asparagus, and cook for about 10 minutes. Stir in the mushrooms and garlic; cook and stir until the mushrooms are tender. Set aside.
Prepare the hollandaise sauce according to package directions. Stir into the asparagus and mushrooms.
Lay one sheet of puff pastry out flat in the bottom of a 9x13 inch baking dish, letting any extra dough go up the sides. Spread the asparagus mixture evenly over the dough. Top with the other sheet of pastry, and pinch the edges together to seal.
Bake for 25 to 30 minutes in the preheated oven, until pastry is golden brown. Cool for a few minutes before slicing and serving.