Wash and dry the four small oranges. Slice two of them. Then, zest one of the remaining whole oranges and juice them both.
Line a baking dish or sheet pan with parchment paper and arrange the orange slices in a single layer.
If your ham has a layer of fat on top, you can trim it off before scoring. This helps the glaze stick better and more evenly infuses the ham with flavor.
Using a sharp knife, lightly score the top of the ham in a crisscross diamond pattern, about 1/4 to 1/2 inch deep. It will help the glaze penetrate and give the ham a classic glazed look.
In a small saucepan, combine the fresh orange juice, honey, apple cider vinegar, stone-ground mustard, orange zest, garlic powder, optional cinnamon, and salt.
Let the glaze simmer over medium heat for 5-7 minutes until slightly reduced.
Place the scored ham with the flat side down on top of the orange slices. Then spoon half of the glaze over the ham.
Thoroughly cover with foil and bake the ham at 325°F for 15-20 minutes, until heated through or until a meat thermometer reads 140°F.
Remove the foil and spoon the rest of the glaze over the ham. Then, bake the ham for 5-10 more minutes. If desired, broil the ham for 2-3 minutes for a beautiful caramelized finish, just keep a close eye on it so it doesn’t burn!
Let the ham rest for about 10 minutes before slicing and serving.