Preheat oven to 375°F / 190° C.Place pork loin on a cutting board. Using a sharp knife, slice through the top third of the pork loin, stopping about 1 inch from the end. Fold that piece back, then rotate the pork loin 180° - this makes it easier to use your dominant hand. Make your second cut, dividing the rest of the pork loin, into 3 even sections. Open it up (like a brochure), and if necessary, pound it out to an even thickness. Cut 8 - 10 pieces of butchers twine, and set aside. Season the pork with half of the salt and pepper.
In a large sauté pan, over medium low heat, melt butter. Cook the celery, onions and garlic until softened, about 6 - 8 minutes. Remove from heat and set aside.
In a medium mixing bowl, combine chopped chestnuts, panko, egg, sage, parsley, and the remaining half of the salt and pepper. Add onion mixture and stir to combine.
Spread stuffing evenly over pork loin, leaving an inch space around the edges. (Your stuffing with get pushed out if you go all the way to the end.)
Arrange pork loin so a short end is closest to your body. Roll the loin up like a pinwheel, as tightly as you are able to, and tie a piece of twine around the roast every inch or two to secure.*Some of the filling will fall out as you are rolling - stuff it back in, it will be fine, I promise. Heat the same sauté pan to medium high heat (remove any bits of onion and celery, so they don't burn) and brown the pork loin on all sides - about 8 minutes total.
Roast, fat side up, at 375°F (190°C) for 45 minutes, or until a thermometer registers 135°F. Transfer pork loin to a cutting board, loosely tented with aluminum foil, and let it rest for 15 minutes. Carefully cut string and remove. Slice and serve immediately.