Preheat the oven to 400 F. Take the pork tenderloin out of the refrigerator and pat dry with paper towels. Brush a thin layer of dijon mustard over the surface of the pork then season generously with salt and black pepper pressing to adhere it to all sides. Let stand at room temperature for 30 minutes.
In the meantime coarsely crush the whole peppercorns with a mortar and pestle or place in a resealable plastic bag and crush with a small saucepan or rolling pin. Set aside.
Heat the oil in a large oven-safe skillet over medium-high heat. When the oil starts to shimmer, add pork to skillet. Cook, turning occasionally, until the pork is browned on all sides, about 8 minutes.
Transfer the skillet to the oven. Roast until a meat thermometer inserted in thickest portion of pork registers 130°F, about 5-7 minutes then reduce the oven's temperature to 200°F and continue to cook until the thermometer registers 140°F, about 5 more minutes. Transfer the pork to a sheet pan and cover loosely with foil. Reserve the skillet to make the sauce. *See recipe card for more notes on internal temperature options.
Place skillet back on the stovetop over medium. Add the butter, shallot, garlic, crushed peppercorns and green peppercorns (if using) to the pan and cook, stirring often, until the shallot and garlic are soft and aromatic, about 5 minutes.
Next, add the sherry/brandy to the pan. Stir with a wooden spoon, scraping up any browned bits from the bottom of the pan. When the alcohol is almost completely evaporated add the stock and cream and simmer on low for 8-10 minutes until the sauce is reduced enough to coat the back of spoon. Season, to taste, with salt.
Slices the pork into 1/4 rounds and transfer to a platter. Pour any juices from cutting board back into skillet and stir into sauce. Spoon the sauce over the sliced pork and serve right away!