1. Preheat oven to 400 degrees.

2. Using a mortar and pestle or a spice grinder, coarsely grind peppercorns, salt, and red-pepper flakes together until peppercorns are broken but not completely ground.

3. Brush a griddle with olive oil (or place oil in a large skillet) and heat over medium-high heat. Season lamb all over with pepper mixture. Place lamb on griddle, meat-side down, and sear to create a golden-brown crust; turn and repeat process on opposite side, about 3 minutes per side. Transfer lamb to a baking sheet.

4. Transfer baking sheet to oven and cook until internal temperature reaches 122 to 125 degrees on an instant-read thermometer, 12 to 15 minutes, for medium rare. Let stand 5 minutes before slicing and serving with chimichurri.