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.
Make-ahead tips: Freeze uncooked biscuits for a couple of weeks; cut out dough rounds, and freeze flat on a baking sheet. Once frozen, transfer to a zip-top freezer bag. Arrange frozen dough on a baking sheet, and allow to thaw slightly as oven preheats. Bake an extra two or three minutes or until browned. You can also assemble the sandwiches about an hour or two before the party; wrap and keep warm at 150°. And just know that these are delicious at room temperature--they don't have to be piping hot.