Preheat your oven to 400°F (200°C). Let the lamb racks come to room temperature—about 30 minutes on the counter. Cold meat straight into a hot pan is a recipe for uneven cooking.
Season the lamb generously. Mix together the minced garlic, rosemary, thyme, 1 tablespoon of olive oil, salt, and pepper. Rub this mixture all over the lamb, focusing on the meaty portions. Let it marinate while you prepare the crust.
Make the pistachio crust. In a food processor, combine the pistachios, breadcrumbs, lemon zest, parsley, garlic, salt, and red pepper flakes (if using). Pulse until the mixture resembles coarse crumbs but still has some texture—you're not making pistachio butter here. Add the tablespoon of olive oil and pulse a few more times to combine.
Sear the lamb to perfection. Heat the remaining tablespoon of olive oil in your oven-safe skillet over medium-high heat. When hot, add the lamb racks, meat-side down. Sear until golden brown, about 2-3 minutes. Flip and sear the other side briefly, about 1 minute.
Apply the Dijon. Remove the skillet from heat and brush the meaty side of each rack with a thin layer of Dijon mustard. This not only adds flavor but helps the pistachio mixture stick.
Add the pistachio crust. Press the pistachio mixture firmly onto the mustard-coated side of the lamb, creating an even layer. Some will fall off, and that's okay—just scoop it up and press it back on.
Roast to perfection. Place the skillet in the preheated oven, with the crust facing up. Roast for about 18-22 minutes for medium-rare (internal temperature of 125-130°F/52-54°C). Adjust timing based on your preferred doneness and the size of your racks.