Place the ham in the stoneware. In a small mixing bowl, combine the allspice, brown sugar, mustard and 2 teaspoons of the lemon juice. Spoon the mustard sauce evenly over the ham. Cover and cook in the slow cooker heating base on Low for 6 to 7 hours, or until the ham is warm throughout and the sauce is well absorbed in the ham. Transfer the ham to a warm serving platter. Transfer the juices from the stoneware to a small, heavy saucepan along with the lemon juice, sugar, and cornstarch. Heat this over medium high heat until the mixture boils. Once boiling, reduce to medium heat, stirring, until the sauce is thickened and glossy. Carve the ham into slices and spoon the sauce over the individual servings.