Mushroom saute (see recipe)
sheet of frozen puff pastry
Add-purpose flour for dusting
cup finely grated fontina cheese (1¾ oz.)
While you prepare the mushroom saute (skip the deglazing), thaw one sheet of frozen puff pastry at room temp. until pliable, 30 to 45 minutes. Set the saute aside to cool. Position rack in the center of the oven and heat the oven to 450 degrees. Lightly dust a work surface with flour. Unfold the pastry sheet and roll it into a rectangle that's aobut 10x 15 inches and about 1/16 inch thick. Slide in onto a baking sheet lined with parchment. With the exception of about an inch border around the rectangle, prick the pastry all over with a fork. With a pastry brush, brush the beaten egg over the border (you won't need all of it). Bake until the pastry begins to puff and the surface feels dry, about 5 minutes. Scatter the mushrooms onto the pastry, leaving the in or so border uncovered. Bake until the crust border is puffed and deeply golden brown, about 10 minutes. Scatter cheese over the mushroom and continue baking until the cheese melts, until 2 to 3 minutes. Let cool briefly on the rack before slicing and serving.