Preheat oven to 350°. Pat pork tenderloin dry. Pour peppercorns into a resealable plastic bag and coarsely crush them with the back of a chef's knife. Season pork generously with 1 1/2 tsp. salt, then coat with crushed peppercorns.
Heat 1 tbsp. oil in a large ovenproof frying pan over high heat until hot but not smoking. Add pork and cook, turning over occasionally, until all sides are browned, 6 to 9 minutes. Transfer pan to oven and cook, turning over once, until an instant-read thermometer inserted in center registers 135°, 8 to 10 minutes. Remove and let rest about 5 minutes.
Meanwhile, peel beets with a vegetable peeler and coarsely grate with a food processor or box grater. Put in a medium bowl and add lemon juice, remaining 2 tbsp. oil, the parsley, and salt to taste; toss well.
Slice pork into 1/2-in.-thick pieces. Divide among 4 plates and serve with slaw.