In a small dry skillet, heat the peppercorns over medium heat until fragrant, 4 to 6 minutes. Transfer to a pan large enough to hold the racks and add the mustard, soy sauce, red wine, thyme, garlic, and oil. Mix, add the racks, and turn to coat them. Marinate in the refrigerator, turning occasionally, at least 8 hours and preferably overnight. Prepare an outdoor grill or preheat the broiler. Season the lamb with the salt and pepper and grill over high heat, or broil, turning once, until brown and crisp, 12 to 15 minutes. Let rest 3 minutes before slicing.