Add remaining 1 teaspoon oil to the pan. Add onion (or shallot) and cook, stirring, until softened, about 30 seconds. Add wine and bring to a boil, stirring. Boil until reduced to about 1/3 cup, 5 to 6 minutes. Dissolve cornstarch in lemon juice and whisk into the sauce. Cook, stirring, until thickened and glossy. Remove from heat and stir in jelly, sugar (if using) and mustard. Taste and adjust seasonings with more salt and pepper, if needed. Spoon the sauce over the pork.