In a glass or ceramic dish, combine the soy sauce, balsamic vinegar and lemon zest. Add the lamb racks, fat side down, and marinate at room temperature for 2 hours, turning once.

Preheat the oven to 450°. Remove the lamb racks from the marinade and pat dry. Season with salt and pepper. In each of 2 large skillets, heat 2 tablespoons of the vegetable oil until shimmering. Add the racks, fat side down, and cook over moderately high heat until nicely browned, about 3 minutes; turn and sear the meaty side of the racks, about 2 minutes longer.

Transfer the racks to a rimmed baking sheet, fat side up. Roast in the upper third of the oven for 15 minutes, or until an instant-read thermometer inserted in the thickest part of each rack registers 120° to 125°(for medium rare meat). Let the lamb rest for 10 minutes before carving.