Preheat the oven to 400?F.
Combine the olive oil, 1/4 cup of the Essence, the chili powder, cumin, salt, and pepper in a small bowl.
Put the lamb on a work surface lined with plastic wrap. With a thin sharp knife, cut a slit lengthwise through the thickest part of the meat, being careful not to cut all the way through to the other side. Gently pry open the flap of meat. Lay another piece of plastic wrap on top of the meat. With a flat meat mallet, pound the meat to an even 3/4-inch thickness. Remove the plastic wrap and season with the remaining 1 tablespoon Essence.
Spread the pesto over the inside of the meat, leaving a 1/2-inch border along the edges. Starting at a short side, roll up the meat tightly, jelly-roll style, to enclose the stuffing. Tie at 1-inch intervals with kitchen twine. Spread the chili rub all over the outside of the lamb.
Place the meat on a rack in a roasting pan. Roast until an instant-read thermometer inserted in the thickest part of the roast reads 125?F, about 1 1/4 hours, for medium-rare.
Transfer the lamb to a platter and let stand for 15 minutes before carving.
Yield: 6 to 8 servings