Preheat oven to 450° F.
In a medium frying pan over medium high heat, sauté butter for about 1 minute. Add shallots and cook until crisp tender (about 5 minutes).
Open and roll out puff pastry dough on a lightly floured piece of parchment, making about a 10 x 16 rectangle. Transfer to a baking sheet. Pat into its surface, leaving a 1 inch border edge.
Sprinkle with chevre cheese. The the prepared shallots and asparagus.
Brush the edge of the tart with egg yolk.
Bake until golden brown and pastry is puffy, about 20 to 25 minutes. Let cool slightly and serve.Makes 4