Preheat oven to 350°F
Wrap defrosted ham in several layers of foil and place in a baking dish
Fill the baking dish with water and place ham in the oven to warm all the way through, approximately 10 minutes per pound
In a saucepan, combine orange slices, sugar and sorrel and boil until it becomes a syrup
Remove ham from oven, discard water and remove foil
Peel away the skin and carve off the desired amount of fat
Using a sharp knife, score the fat of the ham to allow the syrup to penetrate. Pour the sorrel syrup over the top of the ham
Place in the oven to broil, basting occasionally until golden brown
Remove from oven when it has reached the desired colour; allow to rest before carving