Preparation

• Set the lamb in the middle of a large cutting board. Make deep horizontal slices into the thicker parts of the meat so you can open the lamb up like a book and even out the meat’s overall thickness to about 1/2 inch; discard any additional fatty patches. Using a meat mallet or the bottom of a heavy skillet, gently pound the lamb to flatten it slightly and make its thickness even more uniform.
• Put the lamb into a large non-reactive dish. In a small bowl, mix the soy sauce, 2 Tbs. oil, the thyme, and garlic with 1 tsp. salt and 1/2 tsp. pepper. Sprinkle all over the lamb, cover, and refrigerate for at least 4 hours and up to 2 days.
• In a food processor, combine the basil, walnuts, Parmigiano-Reggiano, 3 Tbs. oil, and 1/4 tsp. each salt and pepper; process until the mixture turns into a paste. Transfer to a medium bowl. Sprinkle the lamb with 1/2 tsp, salt, turn it over, and evenly distribute the pesto and breadcrumbs. Roll up the lamb (pesto side in), secure in 5 or 6 places with kitchen twine, and sprinkle with 1/2 tsp. each salt and pepper.
• Position a rack in the center of the oven and heat the oven to 325°F. Heat the remaining oil in a 12-inch (or larger) skillet over medium-high heat. Set the lamb in the skillet and brown the meat on all sides, about 2 minutes per side for a total of 8 minutes.
• Transfer the lamb to a roasting pan and roast in the oven until an instant-read thermometer inserted into the thickest part of the meat registers 135°F for medium-rare, about 40 to 45 minutes. Transfer the lamb to a cutting board, tent with foil, and let rest for 10 minutes. Thinly slice and serve.