Preheat outdoor grill for medium-high heat. Prepare a maple grilling plank by soaking in water according to manufacturer's directions.
Place the pork chops on a clean, flat surface, and cut from one side through the middle to within one-half inch of the other side. Open the two sides and spread them out like an open book. Set aside until needed.
Melt the butter in a large skillet over medium-high heat. Stir in the onion, garlic, and celery; cook and stir until the onion is soft and transparent, about 5 minutes. Add the spinach, and cook until wilted, 3 to 5 minutes. Pour in the chicken broth, and add the croutons: stir until croutons absorb the broth. Remove from heat, and stir in the Italian cheese blend and the Parmesan cheese. Spoon the vegetable-cheese mixture into the pockets of each pork chop. Close one side of the pork chop over the filling. Place pork chops on the prepared grilling plank. Sprinkle with fresh rosemary, and season with salt and pepper to taste.
Cover and grill, turning once, until pork chops are no longer pink in the center and juices run clear, about 30 minutes.