Begin by preparing the marinade. In a small bowl, mix together the grated garlic, finely chopped rosemary leaves, olive oil, Dijon mustard, kosher salt, and black pepper until a thick, aromatic paste forms.

Place the racks of lamb onto a rimmed baking sheet—lining it with aluminum foil will make cleanup easier. Generously massage the rosemary-garlic mixture over both racks, coating them evenly with about ¼ cup of the mixture per rack. Arrange the racks with the rib bones facing downward and let them rest at room temperature for 1 hour. If preparing ahead, you can cover and refrigerate them for up to 12 hours. Be sure to bring the lamb back to room temperature for an hour before roasting.

To cook, preheat your oven to 450°F (232°C). Roast the lamb with the rib bones facing down for 15 minutes. Then, carefully flip the racks so the bones point upward and continue roasting for another 10 minutes, or until the internal temperature of the meat registers 130°F (54°C) for medium-rare.

Transfer the lamb to a cutting board and allow it to rest for at least 10 minutes. This pause allows the juices to redistribute, ensuring moist, flavorful chops.

Using a sharp carving knife, slice between the rib bones to separate the individual lamb chops. For a final flourish, sprinkle with flaky salt just before serving, if desired.