Combine 1 tablespoon mustard, salt and pepper in a small bowl; rub all over pork. Heat oil in a large ovenproof skillet over medium-high heat. Add pork and brown on all sides, 3 to 5 minutes. Transfer the pan to the oven and roast until an instant-read thermometer inserted in the center registers 145°F, about 15 minutes. Transfer to a cutting board and let rest for 5 minutes.
Place the skillet over medium-high heat (take care, the handle will still be hot), add vinegar, and boil, scraping up any browned bits with a wooden spoon, about 30 seconds. Whisk in maple syrup and the remaining 2 tablespoons mustard; bring to a boil, reduce heat to a simmer and cook until the sauce is thickened, about 5 minutes.
Slice the pork. Add any accumulated juices to the sauce along with sage. Serve the pork topped with the sauce.