For the pork tenderloin: Preheat the oven to 425 degrees F.
Place the peppercorns in a resealable bag and crush them coarsely with a meat mallet or rolling pin.
Mix together the salt, peppercorns, thyme, rosemary and crushed red pepper, if using, in a bowl. Season the pork with the seasoning mix.
Heat a large heavy-bottomed skillet over medium-high heat and add the olive oil. Add the pork and cook, turning as needed to brown the pork all over, about 5 minutes. Transfer the skillet to the oven and bake until cooked through (145 degrees F on a meat thermometer), 15 to 20 minutes. Transfer the pork to a cutting board and tent with foil to rest.
For the herby mustard cream sauce: Return the skillet to the stovetop over medium heat. Melt the butter, then add the garlic, thyme and rosemary and cook for a minute or two Sprinkle over the flour and cook, stirring, for an additional minute. Add the cream and bring to a simmer. Simmer until the cream thickens slightly, about 2 minutes. Stir in the whole-grain mustard and parsley. Taste and adjust the seasoning as needed.
Slice the pork and shingle it onto a platter. Spoon some of the sauce over the top and sprinkle over some chopped parsley. Transfer the remaining sauce to a sauceboat or serving bowl and serve alongside the pork.