Finely chop onion and in a measuring cup stir together cider, vinegar, mustard seeds, and mustard.
In a large heavy skillet heat oil over moderately high heat until hot but not smoking and sauté ham until golden and heated through, about 4 minutes on each side. Transfer ham to a platter and keep warm, covered.
In fat remaining in skillet cook onion over moderate heat, stirring occasionally, until golden, about 5 minutes. Add gingerroot and cook, stirring, 1 minute. Stir cider mixture and add to skillet. Boil sauce, stirring occasionally, until reduced to about 2/3 cup, about 5 minutes. Cut butter into 1/2-inch pieces. Add raisins to sauce and whisk in butter and parsley until butter is incorporated.
Pour sauce over ham.