Combine olive oil, garlic, rosemary, 2 teaspoons salt, lemon zest, and pepper in a small bowl. Transfer to a resealable plastic bag and add racks of lamb, turning them until evenly coated with marinade. Refrigerate, turning occasionally, for at least 4 hours and up to overnight. Allow them to come to room temperature before grilling.

Preheat grill to medium. Remove lamb from marinade and lightly pat dry. Sprinkle racks on all sides with remaining teaspoon salt. Wrap a piece of aluminum foil tightly around exposed rib bones (this will prevent them from burning on the grill). Transfer racks to grill, turning occasionally, until a thermometer inserted into the center registers 135 to 140 degrees for medium-rare, 15 to 18 minutes. Set lamb aside to rest for 10 minutes before serving.

After lamb has rested, remove aluminum foil and discard. Cut ribs apart between the bones and serve drizzled with Mixed Herb Pesto alongside Grilled Smashed Potatoes with Feta.

Cook's Note: "Frenched" refers to a technique in which the meat is cut away from the end of a rib or chop so that part of the bone is exposed. If you are uncomfortable doing this yourself, have your butcher do it for you.