DUXELLE (FILLING): Warm a pan with 1 tsp grape-seed oil over medium-high heat. Once warmed, add bacon. Cook until bacon is crisp. Once cooked, remove bacon and set on paper towels to drain. Add onions, 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.
TENDERLOIN: Warm the original pan over mediumhigh heat with oil until on verge of smoking, then reduce heat to medium. Season the tenderloin with salt and pepper as the pan heats up, then add to warmed oil, searing on all four sides evenly for 2 minutes before flipping. After cooking, remove and pat dry with paper towels.
WELLINGTON: Evenly top turkey tenderloin with duxelle, then roll remaining pastry over top of tenderloin, sealing with egg whites on interior bottom edge. Next, brush the ends and top of pastry with egg whites, then seal the ends to create pastry. Bake the sealed pastry on a cookie sheet in a preheated 375°oven for 30 minutes, or until internal temperature of turkey reaches 165°. After cooking remove from oven and allow to rest for 5 minutes, then slice and serve.
SAUCE: Add chicken stock to original pan over medium heat and reduce until ½ cup remains. Once reduced, add fresh parsley, thyme, tarragon, and garlic. Turn off heat and finish with whisked butter and serve.