I really wanted to make a pot pie, but I was feeling lazy and I needed to use my chicken and puff pastry, so I borrowed heavily from Kittencal's http://www.recipezaar.com/149540 "Chicken Wellington" recipe and came up with this.
Begin thawing your puff pastry for around 20 minutes.
Chop your potato, carrots and mushrooms so that they're ready to use.
Season the tenderloins with salt and pepper to taste and then brown them in a non-stick skillet on medium-high heat utilizing your tablespoon of butter, letting them cook for about 2-3 minutes per side, set them aside.
Preheat your oven to 375.
Put a few cups of water into a pot and boil your carrots and potatoes for around 10 minutes. Drain.
In the same skillet you used to make the chicken, dump in your mushrooms, some rosemary and the garlic, stir until you are satisfied, then transfer your potatoes, carrots and the cheese into the skillet, stir, cover and remove from heat.
Lightly flour enough surface area in your kitchen to unfold the puff pastry and do so, use water to dampen the edges of it.
Coat the chicken tenderloins with the spicy mustard, then set 2 each end to end lengthwise onto the pastry, then spoon your cheesy potato mushroom garlic mixture onto the top of the chicken, fold one edge over the mixture, then the other. Crimp the edges and transfer to a baking sheet, apply your eggwash and then make a few slits into the upper layer of pastry.
Put into the oven and time for around 20-30 minutes, depending on how massive it ended up, take out when golden brown and delicious.