Generously season the lamb racks all over with salt and pepper. Heat the avocado oil in a large skillet over medium-high heat, then sear the lamb on all sides for 2-3 minutes until browned. Transfer the lamb racks to a cutting board to cool.
Meanwhile, combine the pistachios, parsley, garlic, lemon zest, paprika, and cayenne pepper in a food processor and pulse until well blended.
Spread 1 tablespoon of mustard over each lamb rack, then coat with the pistachio mixture, pressing down firmly so it adheres.
Place the lamb, fat side up, on a lined baking sheet and transfer it to the oven. Roast for 15-20 minutes until the internal temperature reaches 130°F for medium-rare.
Allow the lamb to rest for 10 minutes before carving.