Melt butter in a large heavy skillet. Sprinkle the slices with the mustard
and brown sugar if desired. Place them in the skillet and fry over medium heat
until golden brown on each side, about 15 minutes a side.
Remove the slices to a platter and slice them thin.
For gravy, dissolve the cornstarch in milk and add to the skillet. Reduce
heat to low, stir in the parsley and bring to a boil. Make sure all the brownings
are loosened from the pan and stir until thickened, about 5 minutes.