Place the steak on a plate greased with 1 Tbs oil and 1 tsp salt and allow to sit until it defrosts well.
Preheat the oven to 400 F with rack in center position.
Heat a heavy bottomed pan or pot to medium high heat with 1 Tbs oil. Sear steaks on all sides for 30 seconds to 1 minute per side, including the edges. The outer bit has to be a deep brown sear. Keep aside.
Melt the butter in a saute pan over medium heat and toss in garlic, mushrooms and thyme, sweating out the mushrooms a bit, stirring occasionally for 3 to 5 minutes. Then add in the mustard and sour cream and continue to simmer, reducing to medium low until the mixture is not watery and is thick and creamy. Place your puff pastry on a clean work surface and roll it a bit longer and thinner, slicing it down the middle into two square pieces. Divide the mushroom mixture onto each of the two pieces of cut pastry, then place a steak on top of each pile of mushrooms.
Fold up the pastry by pulling up the left and right side up over the steak and to the same with the top and bottom edges, securing with egg wash on the bottom. Cut off excess pastry if needed. Flip the pastry so that the seam is on the bottom and place on a baking sheet. Repeat with the other pastry.
Brush both pastries with egg wash all over.
Bake at 400F for 10 minutes then rotate the pan for even baking and browning. Bake for an additional 8 to 10 minutes until the pastry is a golden brown and baked all over until crusty.
Serve warm with shredded cheese or a salad if you wish.
