Preheat oven to 425 degrees.
In a small bowl, combine 1 1/2 tablespoons of mustard with the salt and pepper; rub over entire surface of pork. Heat the oil in a large ovenproof skillet over medium-high heat; add pork and brown on all sides, 3 to 5 minutes total.
Place skillet in the oven and roast for 15 minutes or until an instant-read meat thermometer inserted in the center registers 145°F. Transfer tenderloin to a cutting board and let it rest for 5 minutes. Place the skillet on the stovetop over medium-high heat (careful - the handle will still be hot!); add vinegar and bring to a boil, whisking up all of the browned bits from the bottom of the pan.
Whisk in maple syrup and remaining mustard; bring to a boil then reduce heat and simmer until sauce has thickened, about 5 minutes. Slice the tenderloin and add any accumulated juices to the sauce along with the chopped sage.
Serve sliced pork topped with the sauce.