While the pork is resting, heat vinegar in the skillet over medium-high heat. Boil, scraping up any browned bits with a wooden spoon, about 30 seconds. Whisk in the remaining 1 tablespoon mustard and maple syrup. Bring to a boil, reduce heat to a simmer and cook until the sauce is thickened, about 3 minutes. Add any accumulated juices from the pork to the sauce, along with sage. Serve the pork topped with the sauce.