When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to High and preheat, lid closed, for 10 to 15 minutes.
Pat the lamb dry with paper towels and generously season each rack of lamb with herbs de Provence, salt and black pepper.
Peel carrots, cut into 1 inch pieces and add to a large mixing bowl. Add the potatoes, olive oil, salt, pepper, garlic, and thyme. Stir to combine.
Place a large cast iron skillet on grill and add 1 tablespoon of vegetable oil. Close the lid and allow to preheat for 20 minutes.
Place lamb in skillet and cook, browning on all sides, 6 to 8 minutes. Transfer lamb to a baking pan leaving skillet on grill and set aside.
Stir pistachios, bread crumbs, butter, olive oil and a pinch of salt and black pepper in a bowl.
Spread mustard on the fat-side of each rack of lamb. Pat pistachio mixture on top of mustard.
Place racks of lamb directly on grill grate next to skillet. Add the seasoned potatoes to the skillet. Close lid and cook for 15 minutes.
After 15 minutes, open the grill and stir the potatoes and carrots. Cover the racks of lamb loosely with foil.
Continue cooking for another 5 to 10 minutes or until a thermometer inserted diagonally into the thickest part of the meat registers an internal temperature of 125 degrees F.
Remove the lamb from grill with foil still intact and allow to rest for 10 minutes. Check potatoes with a fork to see if they are tender. If not, allow the potatoes and carrots to cook for another 5 minutes or until tender.
Cut each rack of lamb into 4 double chops and serve with roasted carrots and potatoes. Enjoy!