For the lamb: Whisk together the pomegranate molasses, mustard, horseradish if using and some salt and pepper in a medium bowl. Let sit for at least 30 minutes to allow the flavors to meld.
Remove the lamb from the refrigerator 30 minutes before cooking.
When ready to cook, heat a cast-iron pan until smoking hot over medium-high heat. Brush the lamb with oil and season liberally with salt and pepper.
Sear the lamb on all sides and cook to medium-rare (135 degrees F on an instant-read thermometer), 8 to 10 minutes. Right before removing the lamb from the heat, brush all over with some of the glaze and allow it to caramelize slightly. Remove the lamb to a cutting board and let rest 5 minutes.
For the relish: Combine the pomegranate seeds, parsley, pistachios, onion, vinegar and honey in a medium bowl; season with salt and pepper.
Slice the lamb into medallions, drizzle with more of the glaze and spoon the relish over.