Preheat the oven to 375˚F.
Drizzle some oil in a large, ovenproof skillet set over medium-high heat.
Sprinkle the tenderloins with salt and brown on all sides, turning with tongs.
Meanwhile, stir the maple syrup, balsamic vinegar and grainy mustard in a small bowl. Tuck in the sprig of rosemary.
Pour the sauce over the tenderloins (if you don’t have an ovenproof skillet, transfer them to a baking dish and pour the sauce overtop) and bake for 15-20 minutes, or until the pork feels more firm to the touch and the internal temperature reads 165˚F.
Let them rest for 10 minutes on a cutting board, loosely covered with foil.
As they rest, simmer the remaining sauce on the stovetop until it reduces and thickens, and brush over the tenderloins before slicing. (Discard the rosemary stem.)