To create duxelles, over medium-high heat warm a pan with 1 teaspoongrapeseed oil. Once warmed, add bacon.Cook until bacon is crisp, 3-4 minutes. Once cooked, remove bacon and add onions, again cooking for 2-3 minutes then add mushrooms and butter. Cook onion-mushroom mixture for 2 minutes then add thyme and parsley, season to taste with salt and pepper. Once herbs have been added, cook for final 2 minutes, then remove and drain excess liquid from mushrooms. Once drained, add bacon, again taste and re-season if needed.
For the tenderloin, in the same pan, again warm over medium high heat with oil until verge of smoking then reduce heat to medium. During heating, season tenderloin with salt and pepper then add to warmed oil, searing on first side for 2 minutes then flip and repeat process on all 4 sides. The tenderloin should be evenly seared on all sides. After cooking remove and pat dry with paper towels, then lay flat on bottom side of puff pastry.
Next to prepare wellington, evenly coat on all sides of turkey tenderloin with dijon then top with duxelle, then roll remaining pastry over top of tenderloin, sealing with egg whites on interior bottom edge. Next brush ends and top of pastry with egg whites, and seal ends to create sealed pastry. Next bake in pre-heated 375°F oven, on a cookie sheet for 30 minutes or until internal temperature of turkey reaches 165 degrees. After cooking remove from oven and allow to rest for 5 minutes, then slice and serve.
To create sauce, over medium heat in original pan, add chicken stock and reduce until 1/2 cup remains. Once reduced taste and season, then add fresh parsley, thyme and tarragon, then turn heat off and finish with whisked butter and serve.