Preheat oven to 450°F with rack in middle position.
Cut tenderloins into 6 serving pieces, pat dry, then sprinkle with 1 1/4 teaspoons salt and 3/5 teaspoons pepper (total). Stir together mustard, zest, juice and garlic.
Heat oil in a 12-inch heavy oven proof skillet over medium-high heat until it shimmers, then sear pork until browned on 1 side, about 5 minutes. Turn pork over and spoon mustard mixture over browned sides.
Transfer skillet with pork to oven and roast until pork reaches an internal temperature of 145°F, 12 to 15 minutes. Remove skillet from oven and let pork stand, in skillet, 5 minutes.