Heat oil in a medium skillet over medium-high heat. Season pork with salt and pepper, transfer to the pan and cook until browned and no longer pink in the middle, 1 to 2 minutes per side. Transfer to a plate and cover with foil to keep warm. dd shallot to the pan and cook, stirring constantly, until softened, 30 seconds to 1 minute. Add wine and bring to a boil, stirring constantly. Boil until reduced to about 2 tablespoons, 2 to 3 minutes. Whisk cornstarch and vinegar in a small bowl; whisk into the sauce. Cook, stirring, until thickened and glossy, about 30 seconds. Remove from the heat and stir in jelly, sugar (if using), mustard and any accumulated juices from the pork. Serve the pork with the sauce.