Preheat the oven to 140 degrees C (280 degrees F).
For the rub: Mix together the mayonnaise, olive oil, whole-grain mustard, mustard powder, mint, garlic, lemon zest and juice and some pepper in a small bowl.
For the lamb: Make incisions (small cuts) all over the leg of lamb with a sharp knife. Stuff the sliced garlic with some of the rosemary and thyme sprigs into the incisions. Put the remaining rosemary and thyme in the base of a roasting or baking pan and place the leg of lamb on top. Spread the rub all over the lamb. Pour the stock and about 100 milliliters (1/2 cup) water into the roasting pan around the meat, cover with tin foil and seal.
Roast the lamb until tender, 3 1/2 to 4 hours. Increase the temperature to 180 degrees C (360 degrees F), remove the tin foil and roast until golden, about 30 minutes. Transfer the lamb to a serving board to rest while you make the gravy.
For the mint mustard gravy: Spoon off any excess fat from the pan, put the pan onto the hob (stovetop) and bring the juices to a rapid boil. Add the creme fraiche, mustard, honey and mint leaves. Add the stock, season with salt and pepper and simmer briefly. Pass the mixture through a sieve into a gravy boat.
Carve the lamb and serve with the gravy.