First I removed the lamb from the bag and rinsed. The leg of lamb I bought was already de-boned butterflied, and rolled in a mesh. But if you can't find one already prepared like this, you can ask you butcher to prepare a leg of lamb for you or remove the bone yourself. Once rinsed, skewer the lamb and make sure it is secure. Balance the skewer according to your manufactures directions. Let the lamb come to room temperature as you prepare the other ingredients.
To prepare the mustard mixture, in a large bowl, add the Dijon mustard, red wine, garlic, rosemary, and pepper and mix thoroughly. Divide the mixture in half and set aside one half. Slather the lamb in the mustard mix. Anything that drips off you can add after you get the lamb going on the grill.
Prepare your grill for the rotisserie. I remove my grates to give more clearance. Heat the grill on high. Once nice and hot, about 8 minutes on my grill add the lamb and start the rotisserie turning. Reslather any of the mixture from the initial slathering that ran off the lamb (hopefully you had a plate under the lamb).
With an open lid, roast on high for 10 to 15 minutes until you have a golden brown crust on the lamb.
Now it's time to slow things down. I turned off my center burner and left on my front and back burner on low. Add a pan of water under the lamb, with just enough water to cover the bottom of the pan. Reduce the temperature down to low, about 300 to 325° F. If your grill has left, right and center zones, turn off the zone that is directly under your lamb. If you have charcoal, you'll need to rearrange the coals so they are not directly under the lamb. The pan of water will catch drippings which prevent flareups, regulate the temperature and will help keep the meat moist.
As usual, the amount of time you have to cook the lamb will vary greatly depending on the size of the lamb, your grill, and numerous other factors. Mine took about 2 hours (about 1/2 hour per pound @ 325° F). When the internal temperature gets to 145° F (for medium rare), turn off the grill and carefully remove the pan of water.
Slather the lamb with the remaining mustard. Relight the grill, and put it on medium high, with the middle burner going again. We want some direct heat to get the crust back on the lamb (you'll find that the steam from pan of water may have caused your crust to drip off). After about a minute, add the chopped pistachios. I used a spoon to press them into the mustard and get them to stick, slowly rotating the lamb as I did this.
Cook for another 10 minutes until the crust is brown and crispy.
Remove from the grill.
While it is resting for 10 minutes, take lots of pictures until you get one that looks just right. Well, that's what I did anyway.
Then carve it up and serve.