In a mixing bowl combine cranberry sauce, catsup, orange marmalade, orange juice, and onion. Cover and chill in the refrigerator for up to 24 hours.
Arrange turkey slices in a 2-quart rectangular baking dish. Cover with foil; chill in the refrigerator for up to 24 hours. (Chill sauce and turkey separately.)
Before serving, bake the turkey, covered, in a 350 degree F oven for 25 to 30 minutes or until heated through.
Meanwhile, in a medium saucepan heat the cranberry mixture until hot. To serve, use a slotted spatula to transfer turkey to a warm serving platter. Spoon some of the sauce over the turkey slices on the platter. Serve remaining sauce in a bowl. Makes 12 servings.