Place onion, garlic and parsley in a food processor and place on lowest setting while drizzling olive oil until the mixture becomes paste, like, sort of like a “Pesto.” Set aside.
In a pan, add balsamic vinegar, honey, Worcestershire and salt and pepper. Place on medium-high heat and bring to a slow, rolling boil. Keep mixture at a rolling boil and stir occasionally and let the liquid reduce to about half the volume and can coat the back of a spoon. Continuously taste to make sure it doesn’t burn and adjust salt content at the end. It still may be a bit watery, but it will thicken as it cools. Take off heat and allow to cool. Once cool, combine with the garlic, onion mixture and place in a bag with your lamb. Allow to marinate overnight. Make sure to coat the lamb with remainder of reduction before roasting OR if doing the “Sous Vide” method, coat the lamb before browning at the end.
Links to our lessons for cooking rack of lamb and leg of lamb can be found below.