Preheat the oven to 325 degrees. Spread the pine nuts on a baking sheet and toast until golden, about 5 minutes. Let cool.

Fill a small bowl with ice water. In a small saucepan of boiling water, blanch the basil leaves for 10 seconds. Drain and immediately transfer to the ice water. Drain and squeeze dry. Place the basil in a food processor with the 1/3 cup of warmed oil and puree until fairly smooth. Add the toast, 1 of the garlic cloves, 2 teaspoons of the lemon zest and 1/2 cup of the pine nuts and pulse until coarsely chopped. Season with salt, pepper and cayenne.

Set the lamb on a work surface fat side down and season it with salt and pepper. Rub half of the basil mixture on the meat. Roll up the meat and tie it at 1-inch intervals with kitchen string.

In a flameproof roasting pan, heat the 1 tablespoon of olive oil. Season the lamb with salt and pepper. Add the lamb to the roasting pan and cook over high heat, turning occasionally, until browned all over, 10 minutes. Add the butter, thyme and the remaining garlic clove to the roasting pan and cook for 1 minute, basting the lamb.

Transfer the lamb to the oven; roast for 1 hour and 15 minutes, turning once or twice, until an instant-read thermometer registers 130 degrees for medium-rare. Transfer the lamb to a cutting board; remove the strings.

Preheat the broiler. Spread the remaining basil mixture over the roast and return the meat to the pan. Broil it 8 inches from the heat for 5 minutes, until the crust is lightly browned and sizzling. Transfer the meat to a cutting board; let rest for 20 minutes.

Meanwhile, in a bowl, whisk the lemon juice with the mustard. Whisk in the remaining 1/3 cup of the oil, 1/4 cup of pine nuts, 1 teaspoon of lemon zest and the chopped basil; season with salt and pepper. Slice the meat; transfer to plates. Drizzle with the vinaigrette and serve.