Cut pork loin as described above, and pound out to 3/4" thickness by placing a sheet of plastic wrap over the pork and pounding with meat mallet.

Cut 6 pieces of butchers twine, 8 - 10 inches long, and set aside.

In a medium saute pan, over medium heat, melt butter.

Add onions, celery and garlic.  Cook until softened, about 6 - 8 minutes.

In a medium bowl, combine chopped chestnuts, panko, egg, sage, salt and pepper.

Add onion mixture and stir to combine.

Spread stuffing evenly over pounded out pork loin, leaving a 2 inch space at the end.  (Your stuffing with get pushed out if you go all the way to the end.)

Roll the loin up like a pinwheel and tie to secure.

Heat saute pan to medium high heat and brown pork loin on all sides, 8 - minutes total.

Roast at 375°F (190°C) for 50 minutes, or until thermometer registers 155°F