Combine syrup and vinegar in a small saucepan; bring to a boil. Cook until reduced to 1/3 cup (about 3 minutes), stirring occasionally. Remove from heat; stir in mustard.
Cut pork crosswise into 8 pieces. Place each pork piece between 2 sheets of heavy-duty plastic wrap; pound to 1/4-inch thickness using a meat mallet or small heavy skillet. Heat oil in a large nonstick skillet over medium-high heat. Sprinkle pork evenly with salt and pepper. Add pork to pan; cook 3 minutes on each side. Add vinegar mixture; cook 1 minute or until desired degree of doneness, turning pork to coat. Place 2 pork medallions on each of 4 plates; drizzle about 1 tablespoon syrup mixture over each serving.