Mince the shallots.
Heat your saucepan over medium-high heat, add the oil and let it get hot but not smoking.
Add the shallots and cook until they are a golden brown but pay attention and keep stirring so they don't burn and you have to start all over again. Should take about 4 to 5 minutes.
Add the mustard and sherry vinegar.
Reduce this combination by half, which takes only about 1 minute.
Add the brandy or Cognac and cook to burn off all the alcohol. About 3 minutes. I like to remove the pan from the flame on the stove top before adding any liquid that contains alcohol and can potentially blow up in my face. It shouldn't happen if you remove the pan, but be careful.
Next, add the red wine and port and reduce again by half. This should only take about 5 to 10 minutes since you are reducing a lot less liquid. You want the sauce to be simmering so if starts to boil, reduce the heat to medium or medium-low.
Add the demi glace and continue reducing until the sauce is the consistency you like.I use a spoon to check for the right consistency. When it is thick enough to coat the back of the spoon, I'm there.
When the sauce is reduced to your liking, remove it from the heat and add the cream, butter and crushed peppercorns.
Taste and season for salt. It should be a good consistency at this point but you can always reduce it a little more if you like.
The whole process should take about 20 minutes or 30 minutes less than the "longer" version and in my opinion will have a lot more flavor by using a real demi.