Add the onion, carrot, celery and remaining thyme and rosemary to the skillet and return racks to the pan fat side up. Roast dozen to 15 minutes longer for medium rare, then transfer racks to a work surface and place the skillet over medium-high heat. Add white wine to the skillet and reduce slightly, scraping up any browned bits in the pan. Add the stock and cook until very lightly thickened (about 15 minutes.) Carve lamb into individual chops and arrange on heated serving plates. Strain stock mixture; degrease and season to taste with salt and pepper. Pour the sauce over the lamb and garnish with watercress sprigs. Serve immediately.