Melt butter in a large skillet.
When hot, saute ham on both sides until brown.
Remove and keep warm on a platter.
Add orange sections to fat in the pan and heat for 2 minutes.
Remove to platter with ham.
Mix together the orange juice, mustard, brown sugar, and vinegar.
Add to pan and cook until reduced to half.
Season to taste.
Arrange oranges on top of ham, and pour the few tablespoons of Sauce Over.