Preheat the oven to 375°F. Line a 12 x 18-inch baking sheet with parchment paper and sprinkle with water.

On a lightly floured surface, roll out the pastry into a 14 x 5-inch rectangle 1/8-inch thick. Cut the pastry into 4 rectangles. Place them on the prepared sheet about 1½ inches apart to allow for expansion during baking. Cover with plastic wrap and refrigerate for 20 minutes, or until chilled.

Beat the egg yolk with 2 tablespoons cold water. Brush the tops of the chilled pastry lightly with the egg wash. Bake for 20 minutes, or until puffed and golden.

Meanwhile, in a large skillet, melt the butter over medium heat. Add the mushrooms and season with salt and pepper. Cook, stirring from time to time, just until the mushrooms give off their liquid, about 5 minutes. Pour off the cooking liquid into a saucepan. Keep the mushrooms warm. Cook the liquid over low heat until reduced to about 1 tablespoon. Set aside.

In a medium saucepan of lightly salted simmering water, poach the asparagus for 5 to 7 minutes, or until crisp-tender. Drain and keep warm.

Prepare the sauce: Place the wine, orange juice, and vinegar in a small nonreactive heavy saucepan over medium heat and slowly bring to a simmer. Continue to simmer until the mixture is syrupy and reduced to about 2 tablespoons. Remove the pan from the heat and whisk in about 2 tablespoons of the butter pieces. Return to low heat, and whisking constantly, add the remaining butter piece by piece. Whisk in the reduced mushroom liquid and optional demi-glace. Set aside on the warm side of the stove.

Using a serrated knife, carefully slice each pastry in half lengthwise. Place each bottom slice on an individual warm plate. Divide the mushrooms over each, then add the asparagus spears. Coat the asparagus with the sauce. Place the top pastry over the asparagus and serve immediately.