For the stuffing: Heat the oil in a large skillet over medium heat. Add the sliced onions, season with a generous pinch of salt and saute until soft and translucent, about 5 minutes. Add the Worcestershire sauce and red pepper flakes and saute for another minute. Remove from the heat and let cool.
For the marinade: In a medium bowl, combine the soy sauce, maple syrup, Worcestershire sauce, garlic, red pepper flakes and sage and whisk until blended. Set aside while you prepare the pork loin.
For the pork loin: Slice the pork along the bone about 2 1/2 inches down into the loin from end to end, creating a long flap that opens like a book. Season the inside with a generous amount of salt and pepper. Place the cooled sauteed onions inside the pocket along the length of the loin. Tie the loin closed using butcher's twine: Start at one end and tie it securely. Then work on the other end to tie it shut. Work your way toward the center, tying the loin between each rib.
Place the pork loin in a large baking dish or bowl. Pour the marinade over it and use a brush to coat all sides. Cover the dish securely with plastic wrap, swish it all around to ensure the loin is covered with the marinade and let it sit in the refrigerator for at least 4 hours or as long as overnight.
Place the chipotle pepper in a small bowl. Pour 1/2 cup of hot water into the bowl and let it sit to rehydrate for 15 minutes. Once it is rehydrated, give it a rough chop. Reserve the liquid.
Preheat the oven to 400 degrees F.
Heat the oil in a large cast-iron skillet or heavy-bottomed pan over medium-high heat. Remove the pork loin from the marinade, making sure to save the remaining marinade. Sear the loin until nicely browned, 3 to 4 minutes per side. Place the seared loin in a 9-by-13-inch baking dish or roasting pan.
Turn down the heat on the skillet to medium, add the chopped onion and saute for 2 minutes. Add the chipotle and season with salt, pepper and the oregano. Add the white wine, chicken broth, garlic and the reserved chipotle soaking liquid and marinade. Using a wooden spoon, scrape up all the brown bits from the skillet and let the liquid come to a simmer. Pour everything from the skillet over the pork loin. Transfer it to the oven and roast until the internal temperature reaches 150 degrees F, about 1 hour 30 minutes, making sure to baste the loin every 20 minutes or so.
Let the pork loin rest for 10 minutes. Remove the butcher's twine before slicing. Strain the pan juices and serve them alongside the pork.