Finely grate 6 garlic cloves and pick the leaves from 6 fresh rosemary sprigs, then finely chop (4 to 6 tablespoons). In a medium bowl, add 3 tablespoons olive oil, 2 tablespoons Dijon mustard, 1 tablespoon kosher salt, and 1 teaspoon black pepper, and stir to combine.

Place 2 lamb racks on a rimmed baking sheet. Spread the rosemary rub all over the lamb racks (about 1/4 cup each). Position the racks so the rib bones point down. Let sit at room temperature for 1 hour, or refrigerate covered for up to 12 hours; if refrigerated, let sit at room temperature for 1 hour before roasting.

After 30 minutes, arrange a rack in the upper third of the oven and heat the oven to 450?F.

Roast the lamb for 15 minutes. Flip the racks and roast until the thickest part registers 130?F on an instant-read thermometer for medium-rare, about 10 minutes longer.

Transfer the racks to a clean cutting board and let rest for at least 10 minutes. To carve, cut between the rib bones into individual chops. Lightly sprinkle with flaky salt if desired. Place on a serving platter and garnish with more fresh rosemary if desired.