Ready-to-use frozen puff pastry simplifies the preparation of this mushroom soup en croute. (En croute is a French term referring to food that is wrapped or covered with pastry and then baked.) Puff pastry is formed by folding dough repeatedly to create layers with butter between each one. As the dough bakes, the moisture in the butter creates steam, causing the pastry to puff up. Puff pastry is easy to work withjust keep it cold so the butter stays firm. An egg wash brushed on top of the dough before baking produces a shiny, golden brown crust.