Mix marinade ingredients in a sealed plastic bag. Place lamb in bag and coat it in the marinade, then let it sit for 6-8 hours.
Be sure to take lamb out at least 45 min before cooking.
Preheat oven to 400 degrees F.
Heat a tablespoon of oil in a oven-proof skillet on high. Sear one lamb rack on all sides until golden brown, including the ends. Remove and repeat with other lamb rack.
Place both racks in the skillet and roast for 15 minutes (or until reaches 135 degrees internally) and take pan out of the oven.
Add the butter, crushed garlic and rosemary inside. Spread the butter over the lamb.
Take lamb out of the skillet and place on a rack set over a tray. Pour the butter remaining in the skillet over lamb.
Cover with foil and let it sit for several minutes. When finished, serve!
