Preheat oven to 350°F. Line a large rimmed baking sheet with aluminum foil; set aside.

Pat pork dry with paper towels, and season with 2 teaspoons salt and ½ teaspoon pepper; set aside. Whisk together Worcestershire sauce, grated garlic, 1 tablespoon oil, 1 tablespoon mustard, 2 teaspoons thyme, and ½ teaspoon pepper in a small bowl until smooth; set aside.

Heat remaining 2 tablespoons oil in a large stainless steel skillet over high until shimmering, about 3 minutes. Add pork; cook, turning occasionally, until well browned on all sides, 6 to 8 minutes total. Remove from heat, and transfer pork to prepared baking sheet. Brush reserved Worcestershire mixture evenly over pork. Do not wipe skillet clean.

Bake pork until an instant-read thermometer inserted into the thickest portion of pork registers 140°F (for medium), 15 to 18 minutes, or until desired degree of doneness. Remove from the oven, and let rest for 5 to 10 minutes.

While pork is baking, melt 1 tablespoon butter in reserved skillet over medium-low. Add shallot, allspice, cloves, juniper berries (if using), remaining 1 teaspoon thyme, and remaining ¼ teaspoon pepper. Cook, stirring constantly, until shallot is softened, about 1 minute. Add vinegar; cook, scraping bottom of skillet to release any browned bits, until vinegar is nearly evaporated, about 2 minutes. Add apple cider, remaining 1 tablespoon mustard, and remaining ½ teaspoon salt.

Bring to a simmer over medium-high, and cook, stirring occasionally, until sauce thickens and is reduced to about ⅓ cup, about 8 minutes. Remove from heat; discard whole spices. Vigorously stir in chives and remaining 2 tablespoons butter until sauce is glossy and emulsified, about 1 minute.

Transfer pork to a cutting board, and cut into ½-inch-thick slices. Transfer to a platter, and spoon cider sauce over top. Garnish with additional chives.