Start by tossing the garlic, rosemary, salt, pepper, olive oil, lemon juice, and zest into a small bowl. Give the marinade ingredients a good whisk.
Place the boneless leg of lamb on its flat side in a ziplock bag. Pour the marinade into the bag. You can reserve about ¼ of the marinade. We’ll use it later.
Then, seal the bag and roll it around a couple of times until the lamb is coated with the marinade.
If you’re in a hurry, you can move on to the next step. But for an augmented taste, let the coated lamb sit in the refrigerator for one hour to let the marinade flavours fuse nicely.