To prepare the roast, remove any twine or netting around the pork. Locate the cut made to remove the bone, then open up the roast. Using a sharp knife, continue the cut until the roast opens like a book; do not cut all the way through, as the meat must remain in one piece. Using the tip of a paring knife, make 1-inch-deep incisions into the pork, spaced about 1 inch apart; do not cut all the way through the meat. Set aside.

In a food processor, pulse the pancetta until coarsely chopped, about 15 pulses. Add the butter, rosemary, oregano, garlic, pepper flakes, ½ cup of the ground fennel and 1 teaspoon salt. Process until the mixture forms a spreadable paste, about 1 minute, scraping the bowl as needed.

Spread the paste evenly over the interior of the pork, pressing the paste into the cuts. Roll the roast into a tight cylinder, then set it seam side down.

Cut 7 pieces of kitchen twine, each 15 inches long. In a small bowl, stir together the remaining 2 tablespoons ground fennel, 1½ teaspoons salt, the brown sugar and pepper. Rub this mixture over the top and sides of the roast. Using the twine, tie the roast at 1-inch intervals, seam side down; you may not need all of the twine. Wrap the roast tightly in plastic, transfer to a large baking dish and refrigerate for at least 24 hours or up to 48 hours.

Heat the oven to 300°F with a rack in the middle position. Set a roasting rack in a roasting pan and pour 4 cups water into the pan. Unwrap the roast and set it fat-side up on the rack. Roast until the center registers 195°F, 6 to 7 hours.

Transfer the roast to a carving board and let rest, uncovered, for 1 hour. Reserve the liquid in the pan.

While the roast rests, make the sauce. Pour the liquid in the roasting pan into a fat separator; if making roasted fennel, do not wash the roasting pan. Let the liquid settle for 5 minutes. In a medium bowl, whisk together ¾ cup of the juices, the lemon juice, water, olive oil, pepper and ground fennel.

Cut the pork into thin slices, removing the twine as you slice. Serve with the pan sauce.