In a bowl, whisk together the mustard, barbecue sauce, olive oil, soy sauce, salt, pepper, paprikas, garlic powder, and Italian seasoning.

Line a baking dish with parchment paper and spread a thin layer of sauce on the bottom. Place the whole pork loin on top and coat thoroughly with the remaining sauce. Wrap tightly in the parchment, then cover securely with aluminum foil.

Roast for 75–90 minutes, until the center reaches 135–140°F/57–60°C. Increase the oven to 350°F/180°C. Uncover, reserving the pan juices. Stir half the juices with 1 ounce/28 grams butter and spoon over the pork. Return to the oven and roast, uncovered, for 20–25 minutes, until the internal temperature reaches 145°F/65°C.

Crush the garlic into the remaining pan juices. Scatter the mushrooms around the pork and spoon the garlic sauce over them. Roast for 8–10 minutes more, until the mushrooms are tender and lightly caramelized and the pork remains at 145°F/65°C.

Let the pork rest for 10–15 minutes before slicing. Serve with the mushrooms and pan sauce.

Meanwhile, place the potatoes in a large pot of salted water and bring to a boil. Cook until fork-tender, about 20 minutes. Drain, reserving some of the cooking water. Mash with the butter, adding warm cooking water as needed until smooth and creamy. Season to taste and serve alongside the pork.