1. 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.
2. 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.
3. 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.
Wine Recommendation: The juicy, ripe fruit of a spicy, full-bodied New World Syrah will complement the intensity of the lamb and balance the salty-sweet notes of the marinade; it will also echo the hints of pepper in the watercress salad that follows. Two good possibilities: the 2000 Columbia Crest Reserve Syrah from Washington State and the more expensive 1999 Rosemount Estates Balmoral Syrah from Australia.