Melt the unsalted butter in a medium saucepan over medium heat. Add the finely minced shallot and cook until softened and translucent, about 2-3 minutes. Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.

Carefully pour in the brandy or cognac (if using). Increase the heat to medium-high and bring to a simmer, scraping up any browned bits from the bottom of the pan with a wooden spoon. Allow it to simmer until the alcohol has mostly evaporated, about 1-2 minutes. (If omitting brandy, skip this step and proceed to adding broth).

Pour in the beef broth and bring it to a gentle boil. Reduce the heat to medium-low and let it simmer for about 3-5 minutes, allowing the liquid to reduce by about a third and concentrate its flavor.

Stir in the heavy cream, lightly crushed green peppercorns, and freshly cracked black peppercorns. Add the Dijon mustard and Worcestershire sauce. Whisk gently to combine all ingredients.

Continue to simmer the sauce over medium-low heat for another 2-3 minutes, stirring occasionally, until it slightly thickens to your desired consistency. It should be thick enough to coat the back of a spoon.

Taste the sauce and season with salt as needed. Remember that green peppercorns in brine can contribute some saltiness, so taste before adding much. Serve immediately over grilled or pan-seared meats, garnished with fresh chopped parsley if desired.