1. Preheat oven to 400°.
2. Heat a large ovenproof skillet over medium-high heat. Rub pork evenly with oil; sprinkle evenly with pepper and kosher salt. Add pork to pan; cook 6 minutes, turning to brown on all sides. Place pan in oven. Bake at 400° for 17 minutes or until a thermometer inserted into thickest portion of pork registers 145°. Remove pork from pan; let stand 10 minutes.
3. Increase oven temperature to 450°.
4. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 3 ingredients (through baking soda) in a large bowl, stirring with a whisk. Cut in butter with a pastry blender until mixture resembles coarse meal. Toss in thyme and sage. Add buttermilk; toss with a fork until a soft, sticky dough forms. With floured hands, gently pat dough out onto a lightly floured surface to a 3/4-inch thickness. Cut with a 2-inch biscuit cutter into 20 rounds, gently reshaping scraps as necessary. Arrange dough rounds on a baking sheet lined with parchment paper. Bake at 450° for 17 minutes or until lightly browned.
5. Combine mustard and syrup, stirring well. Cut each pork tenderloin into 20 thin slices. Split each biscuit in half; spoon about 3/4 teaspoon mustard mixture onto cut side of each biscuit top. Arrange 2 pork slices on bottom half of each biscuit; top with top halves.