In a food processor or blender pulse the nuts to crush them up very small. (be careful not to turn them into a paste)
Preheat the oven to 400°F. Line a sheet pan with foil and coat the foil with cooking spray.
Season the lamb well with salt and pepper.
Heat the oil in a skillet or non-stick pan over medium heat. Once the pan is hot, sear the lamb on all sides in the pan until it gets deep coloring on it.
Rub the Dijon mustard and garlic on the top of the lamb. Press the pistachio evenly over the mustard.
Bake in the oven to your desired temperature using a thermometer inserted into the thickest part of the meat (see below). Lamb is commonly served medium-rare, with an internal temperature of 120°F.
For the Au Jus: In a saucepan over medium heat, bring the broth, rosemary, and butter to a simmer. Let the liquid reduce by about a 1/3. Season with salt and pepper to taste. Then set aside.
Place on a cutting board and let rest for 10 minutes.
Slice the lamb in between the bones. Drizzle with au jus. Serve with bread. Garnish with thyme.