Trim excess fat from the leg of lamb, leaving a thin layer for flavor.
Score the fat cap with shallow cuts in a crisscross pattern.
In a bowl, mix garlic, rosemary, thyme, lemon zest and juice, paprika, olive oil, salt, and pepper.
Rub the marinade all over the lamb, including the scored fat. Coat with Dijon mustard if desired.
Cover and refrigerate for at least 4 hours or overnight for best flavor.
Preheat your smoker to 250°F (120°C) and prepare your wood chips.
Place the lamb on the smoker rack or a roasting pan with a rack inside.
Insert a meat thermometer into the thickest part of the meat, avoiding the bone.
Smoke for 4–5 hours, or until the internal temperature reaches 135°F for medium-rare.
Remove from the smoker, tent with foil, and let rest for 20 minutes before slicing.
Slice against the grain and serve with your favorite sides.