Preheat oven to 325 degrees. Wrap ham with parchment-lined foil. Transfer to a rimmed baking sheet. Bake for 1 hour.
Meanwhile, bring jelly and rosemary to a simmer in a small saucepan over medium-high heat. Cook until thick and syrupy, about 3 minutes. Stir in lemon juice and salt.
Transfer ham to a cutting board, and unwrap. Score fat in a diamond pattern with lines less than 1/2 inch apart using a sharp knife. Return ham to baking sheet. Raise oven temperature to 375 degrees. Brush ham with currant glaze. Bake for 30 minutes. Brush again with glaze, and bake for 10 minutes more. Remove from oven, and brush again with glaze. Rewarm remaining glaze over low heat until pourable and saucelike. Slice ham, and serve with sauce.