In a pan build up the sauce on fonds and drippings
Add the finely diced Shallots until see-through
Add the finely chopped garlic and be careful not to burn it
Once you can smell the garlic transfer everything in a bowl
Add the butter
Add the drained green peppercorn and let them fry until you start smelling the pepper
Once ready, kill the heat and add the Brandy
Flambé them
Once the flames are out turn your stove on to medium heat
Shimmer until the Brandy is almost evaporated
Add the beef broth
Bring it to shimmer and let it reduce
Once the broth has started reducing add the heavy cream
Let it shimmer until right consistency
Once the consistency is right add back the onions and garlic
Stir it up
Add the mustard
Taste it and add salt, if needed
Turn off the stove and once the sauce stops shimmering add 1 tbsp butter
ready to serve