Make the brine: In a saucepan, combine kosher salt, mustard powder, cumin, coriander, caraway, fennel, lemon peel, garlic, and water. Bring to a gentle simmer, stirring until the salt dissolves, then cool to room temperature.
Stir in the buttermilk once the brine is fully cool.
Brine the lamb: Place the leg of lamb in a large non-reactive container or zip bag and pour the brine over to submerge. Refrigerate 24 hours, turning once halfway through.
Roast: Preheat oven to 425°F (220°C). Remove lamb from brine and discard brine. Pat the lamb very dry with paper towels.
In a small bowl, mix olive oil, honey, lemon juice, and freshly ground black pepper. Rub the mixture all over the lamb and set it in a roasting pan. Pour about 2 cups water into the pan.
Cover the pan tightly with foil and roast for 2 1/2 hours.
Uncover, baste with pan juices, and roast another 30 minutes, or until browned and the internal temperature is 135–140°F (57–60°C) for medium (it will rise slightly as it rests).
Rest the lamb 15–20 minutes on a cutting board. Skim excess fat from pan juices and reduce briefly on the stovetop if desired.
Make the relish: Whisk honey, lemon juice, and olive oil in a bowl. Stir in chopped mint and pistachios; season with salt and pepper to taste.
Slice the lamb against the grain, spoon over pan juices, top with mint–pistachio relish, and garnish with feta if using. Serve warm.