In a bowl, mix the garlic powder, mustard seed, turmeric, paprika, chili powder, thyme leaves, and the rosemary. Coat the roast with the mixture, and place it inside a vacuum-sealed bag. Cook the roast in a hot water bath at 131 degrees Fahrenheit for 14 hours.
Preheat the oven to 425 degrees Fahrenheit. Remove the roast from the bag and pat dry. Mix the black pepper, Szechuan peppercorns, rosemary, kosher salt and olive oil together, and rub them over the roast. Place the roast in the oven until a crust forms, about 5 minutes.
Let the meat rest for 3 minutes. Slice and serve.