Rub the pork with the salt and pepper. Set on a rack on a baking sheet and refrigerate for at least 12 hours and up to 24 hours.

Preheat the oven to 175 F.

Combine the oil and mustard seed in a Dutch oven and place over medium heat until the oil reaches 350 F on a deep-frying thermometer. A wooden chopstick or the end of a wooden spoon inserted into the oil will emit bubbles when the oil is at the right temperature.

While the oil is heating, remove the pork from the refrigerator. Pat dry. When the oil is up to temperature, turn off the heat. Using tongs, carefully submerge the meat in the hot oil. Cover the pot, transfer to the oven and cook until the pork is 150 F, about 2 hours.

Lift the pork from the oil onto a baking sheet to catch any oil drips. Carve the pork across the grain on the diagonal into thin slices. Drizzle with some of the mustard-scented oil and serve.