1 package (2 sheets) frozen puff pastry,
defrosted (I use Pepperidge Farm)
2 Tbsp Dijon mustard
1/4 lb thinly sliced turkey breast
1/2 lb thinly sliced Gruyere cheese
1 egg beaten with 1 tsp of water (egg wash)
Preheat oven to 450°F. Line a rimmed sheet pan with parchment paper.
Lay one sheet of pastry on a floured counter top, and roll it out to 10x12 inches. It's very hard to keep the pastry squared on the edges, but do the best you can. You can trim it later. Place pastry on the sheet pan.
Brush the center of the pastry with the mustard, leaving a one-inch border around the edge (leave a larger border if you think you'll have to trim the pastry later). Layer on the turkey breast to cover the mustard, and then spread the cheese.
Brush the border with the egg wash.
Roll out the second sheet of puff pastry to the size of the first, and place it on top of the filled pastry, lining up the edges as best you can. If the edges are a bit ragged, trim with a very sharp knife. Press the edges together to seal them, and then, as extra insurance and for a bit of decoration, press with the tines of a fork all the way around.
Brush the top of the tart with egg wash. With a small, sharp knife, make three slits in the top (cut all the way through the pastry) to allow steam to escape.
Bake on the middle rack of the oven for 20-25 minutes, until puffed and golden brown. Allow to cool for a few minutes, and serve hot or warm. It's also good cold, which makes this a great picnic dish, but the puff will settle down a bit.