Whisk together the jam, mustard, sugar and 1/4 teaspoon black pepper in a small pot. Bring the mixture to a boil to melt the sugar, whisking occasionally. Reduce the heat to a simmer until the mixture thickens, about 2 minutes. Remove from the heat, and reserve.
Add 3/4 cup water to the insert of a 6-quart slow cooker. Place the ham in the insert on its side, and brush lightly with the glaze. Cover, and cook on low until the ham is hot throughout, 6 to 7 hours. Refrigerate the remaining glaze.
When the ham has about 10 minutes left to cook, reheat the glaze in a small saucepan over medium-low heat, stirring frequently.
Transfer the ham to a carving platter or serving dish, and brush with enough glaze to make a glossy, sticky coating. Slice, and serve with the remaining glaze on the side.
Depending on the size of the ham, you may have to trim some from the underside. Toss the trimmed pieces into the slow cooker alongside the ham so it can all cook together.
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