How to Make It
In a blender, puree the garlic, chopped thyme, lavender, juniper berries, honey and oil to a smooth paste. Stir in the lemon zest and bay leaves. On a rimmed baking sheet, rub the marinade all over the lamb. Cover with plastic wrap and refrigerate for at least 6 hours or overnight. Let stand at room temperature for 30 minutes.
Preheat the oven to 325°. In a roasting pan, toss the onions, parsnips, carrots and thyme sprigs with the butter. Season with salt. Set the lamb on the vegetables and roast for about 1 hour and 45 minutes, basting occasionally, until an instant-read thermometer inserted in the thickest part of the leg registers 140° for medium-rare.
Increase the oven temperature to 450° and roast for 10 minutes longer, until the lamb is deep golden brown. Transfer to a cutting board and let rest for 20 minutes. Carve the lamb and transfer to a serving platter. Sprinkle with sea salt and serve with the vegetables.