In a small pot, combine brown sugar and mustard. Bring to a boil and mix. Pour over pastrami.
Remove naval pastrami from bag. Pour sauce on top. Cover very tightly and bake for 6-8 hrs.
This freezes well. Slice and freeze in the sauce. Remove from freezer and warm up (covered tightly) in oven. This can sit in the oven on a low temperature for a long time.