Heat oven to 375 degrees. Using a sharp knife, butterfly the top third of the roast from long end to long end, opening it like a book. Butterfly the remaining two-thirds of the roast in half, opening it the same way to create a flat piece of pork.
Mix together garam masala and 1/2 tsp of the salt. Rub mixture onto the entire fat side of the roast. Turn over and set aside.
Melt 1 tbsp of the butter in a large, oven-safe sauté pan (not nonstick) over medium heat. Add onion; cook 5 minutes until softened. Pour in 1/2 cup of the chicken broth and the dates. Bring to a simmer; cook 2 minutes. Remove mixture to a bowl. Stir in chestnuts, bread crumbs, 1/2 tsp of the salt and the pepper.
Distribute stuffing on the non-rubbed side of the pork, pressing down and leaving a 1-inch border on long ends and a 2-inch border on short ends. Roll tightly from one short end to the other. Tie 4 or 5 pieces of butcher string around pork to secure it.
In the same pan, heat canola oil over mediumhigh heat. Sear pork on one side for 2 minutes. Repeat twice, carefully turning each time. Turn pork once more and transfer pan to oven. Roast at 375° for 45 minutes or until internal temperature reaches 145°.
Remove pork to a cutting board to rest for 10 minutes. Being careful of the hot handle, melt remaining 1 tbsp butter over medium heat. Whisk in flour; cook for 2 minutes. Whisk in remaining 11/4 cups chicken broth. Bring to a simmer and cook until thickened, 2 to 3 minutes. Stir in remaining 1/2 tsp salt.
Slice pork and serve with pan sauce.