Allow the ham to sit at room temperature 30 minutes prior to cooking.
Preheat the oven to 325 degrees F. Place the ham in a roasting pan (uncovered) and bake for 1 hour and 10 minutes.
While the ham is baking, make the glaze by combining the brown sugar, vinegar, garlic and mustard in a saucepan and simmer over medium heat until it is thick enough to coat the back of a spoon, about 10 minutes. This can be made a few hours in advance and reheated before use.
Pour the glaze over the entire ham and continue to cook for an additional 20 minutes or until the ham reaches an internal temperature of 140 degrees F. Serve.
Notes
If preparing this ham as gluten-free, just be sure you're buying a ham that is known to be GF.