Adjust the oven rack to the lower-middle position and heat oven to 325°Line a large rimmed baking sheet with aluminum foil and set a V-rack wire rack inside. Spray the rack with nonstick cooking spray.
Butterfly the pork loin roast or ask your butcher to do this for you.
Use paper towels to pat the pork dry and rub it on all sides with salt and sugar. Place the roast in a zipper-lock bag and refrigerate 1-4 hours.
In a large skillet, cook the sausage and bacon until the sausage has browned, the bacon is cooked through but not crispy. Transfer the meat to a paper towel-lined plate reserving 2 tablespoons of the rendered fat in the skillet.
Add the onion, celery and apple to the skillet. Season with salt and pepper and cook until the onion, celery and apple are tender. Add the minced garlic and remove the skillet from the heat. Pour the onion mixture into a large mixing bowl and wipe out the skillet.
Add the reserved sausage/bacon mixture to the onion mixture, in the bowl, and stir well.
While the above cooks, place the bread, sage, rosemary and thyme in the bowl of a food processor and pulse the mixture into coarse crumbs.
Set the skillet over medium heat and add the butter. When the butter melts, add the coarse breadcrumb mixture and stir until the breadcrumbs are lightly toasted, about 3 minutes (do not allow the breadcrumbs to burn). Add the breadcrumb mixture to the sausage/onion mixture and stir well. Set aside.
Remove the pork from the bag and set it in a second large baking tray. Pat it dry with paper towels and arrange it, fat cap down, on the tray.
Cover the pork with the sausage/onion stuffing, leaving a ½-inch border on all sides.
Cut 7 (10-inch) pieces of kitchen twine and set them next to the work area.
Roll the pork tightly and tie it at 1-inch intervals along the length of the roast. Season it, on all sides, with kosher salt and black pepper.
Heat the large skillet over medium heat and add 2 tablespoons oil.
Place the roast in the skillet and brown on all sides (about 3 minutes per side).
Set the roast in the V-rack wire rack on the rimmed baking sheet, fat side up, and set it in the oven. Pour the chicken stock under the roast in the bottom of the baking sheet.
Roast the pork 50-60 minutes or until the thickest, middle part of the pork registers 145°F on a meat thermometer. Remove the roast from the oven, tent loosely with foil, and let it rest 10-20 minutes before slicing. Remove the kitchen twine before slicing.
If desired and if there is enough, pour any rendered juices from the bottom of the pan through a mesh strainer and serve alongside the roast.