Preheat the oven to 350 degrees F. Season the pork loin with the Chinese five-spice powder, salt and pepper.
Heat a large saute pan over medium-high heat with the oil until it begins to smoke. Sear the pork on all sides until caramelized, and set aside.
Return the pan to the heat and lower to medium. Add the tarragon, thyme and garlic and cook until the garlic is lightly browned. Add the vermouth and chicken stock and reduce the liquid by half. Stir in the cream, horseradish and mustard, and season with salt and pepper.
Place the roast in the center of a large baking dish or shallow casserole dish (large enough to fit it comfortably with room on the sides). Place the apples and onions in the dish, along with the tarragon sprigs on top of the vegetables. Spoon the liquid from the pan over the roast and place in the oven.
Cook the roast until the internal temperature reads 150 degrees F, about 12 minutes per pound. Allow to rest for 15 minutes before serving.
Top with the Gremolata and serve.
Combine the parsley, olive oil, pistachios, breadcrumbs, garlic, lemon zest and some salt and pepper in a bowl.