Generously season rack of lamb with salt and pepper.
In a large bag, add lamb and olive oil, garlic, and lemon juice, mustard and lemon zest. Let marinate at least two hours or overnight.
Preheat oven to 180 C (350 F).
Heat a heavy cast iron pan on the stovetop at medium-high heat.
Remove lamb racks from marinade, pat with towels to remove excess oil and liquid (doesn’t need to be bone dry, just not dripping).
Add racks to pan and brown on each side, about 4 minutes per side.
Move pan to oven and let roast for 15 minutes, fat side down.
Flip racks on other side, and roast an additional 10-15 minutes, until internal thermometer reaches 125 degrees for medium-rare, then remove from oven.
Let stand for 10 minutes before carving.