Coarsely grind the peppercorns in a spice or coffee grinder and transfer to a medium bowl. Add the parsley, mustard, butter and 1 tablespoon salt in a medium bowl and mix together until thoroughly combined. Rub the spiced butter all over the tenderloin, rolling the beef in the portions that fall off to coat completely. The beef can be prepared up to this point, covered, and refrigerated up to 1 day in advance.
Preheat the oven to 450 degrees F.
Place the beef on a rack and set over a baking sheet. Roast until a meat thermometer inserted into the center registers 130 degrees F for medium rare, about 35 minutes. Remove from the oven and let stand for 10 minutes. Transfer the beef to a cutting board, preferably a meat board with a reservoir to catch the juices, and garnish with the parsley.
Savory Horseradish Panna Cotta:
Spray six 4- to 6-ounce ramekins with cooking spray. Stir together the sour cream, Dijon, horseradish and 1 teaspoon salt in a small bowl. Heat the half-and-half in a small saucepan over medium-high heat until it just barely comes to a simmer.
Meanwhile, put the gelatin in a large bowl, add 3 tablespoons warm water and stir until completely dissolved. Gradually pour in the warm half-and-half while whisking constantly.
Add a splash of the half-and-half mixture to the sour cream mixture and stir to loosen, then add the sour cream mixture to the large bowl with the half-and-half mixture, whisking gently until thoroughly combined.
Pour the mixture into the ramekins (the ramekins will not be full) and refrigerate until set, at least 4 hours. The panna cottas can be kept refrigerated, covered with plastic wrap, up to 2 days
When ready to serve, run a thin knife along the edge of each ramekin and invert the pana cottas onto plates. Sprinkle with the chives and serve immediately.