Whisk olive oil, garlic, thyme, parsley, lemon zest, salt, and pepper into a marinade.

Coat the lamb loin in the marinade and refrigerate at least 2 hours or overnight.

Preheat oven to 425F and let the lamb come to room temperature.

Sear the loin in a hot skillet on all sides until browned, about 6 minutes total.

Transfer to the oven and roast 18-20 minutes until it reaches 130F for medium-rare.

Rest 10 minutes, then slice into medallions and serve.