Reduce the heat to medium.
Add the remaining tablespoon of butter to the same skillet.
Add the minced shallot and cook for 1 to 2 minutes, until softened.
Add the garlic and cook for 30 seconds, stirring continuously.
Remove the skillet from the heat for safety before adding Cognac or brandy.
Pour in Cognac or brandy, if using, then return the skillet to medium heat.
Scrape up the browned bits from the bottom of the pan with a wooden spoon.
Simmer for 1 to 2 minutes, until the liquid is reduced by about half.
Add chicken broth, Dijon mustard, optional green peppercorns, and thyme.
Simmer for 2 minutes, stirring until the Dijon is fully blended into the sauce.
Pour in heavy cream and stir well.
Simmer gently for 3 to 5 minutes, until the sauce thickens enough to coat the back of a spoon.
Stir in parsley and optional lemon juice.
Taste and adjust with salt and extra black pepper if needed.