Preheat the oven to 325 degrees F (165 degrees C).
Place ham in a shallow roasting pan. Mix mustard, maple syrup, brown sugar, and onion powder together in a small bowl. Coat ham entirely with the glaze using a spoon or brush.
Roast ham in the preheated oven, uncovered, until heated through, about 2 hours. Let ham rest for about 15 minutes before carving to keep it from drying out.
