Preheat your oven to 375℉. Rinse the pork loin and pat it dry with paper towels.
Place the pork loin on a cutting board, fat side up. Starting on the right side, make a 1-inch deep cut along the length of the loin, parallel to the cutting board, without cutting all the way through. Open the meat like a book and continue making shallow cuts until the pork loin lays flat.
Lay the butterflied pork loin flat on the cutting board with the long edge closest to you. In a small bowl, mix the sage, salt, pepper, garlic powder, and onion powder, then evenly season the pork.
Spread your chosen stuffing in a thin, even layer over the seasoned pork, leaving about 1/2 inch of space at the far edge to help prevent the stuffing from spilling out as you roll.
Starting with the long edge closest to you, roll the pork tightly like a jelly roll, tucking in any escaping stuffing as you go.
Secure the rolled pork loin by tying it with kitchen twine at 2-inch intervals along the length to keep the stuffing in place.
Season the outside of the rolled pork loin with additional salt and pepper. Heat the avocado oil in a heavy-bottomed pan over medium-high heat. Sear the pork on all sides, about 2–3 minutes per side, until browned.
Transfer the seared pork loin to a roasting pan. Roast in the oven for 60–75 minutes, or until the internal temperature reads 145°F on a meat thermometer.
Strain any pan juices left in the roasting pan through a fine sieve into a saucepan. Add 1/2–1 cup of chicken stock to the juices and heat over medium-low, stirring often.
In a small bowl, mix 2 Tbsp. of the liquid from the saucepan with 1 Tbsp. cornstarch or arrowroot powder. Add the mixture back into the saucepan and whisk to combine.
Continue heating until the sauce thickens. Season with additional salt and pepper to taste.
Remove the pork loin from the oven and transfer it to a plate. Tent loosely with foil and let it rest for 5–10 minutes to retain juices.
Place the rested pork loin on a cutting board, remove the kitchen twine, and slice into 1/2-inch thick portions. Serve warm and enjoy!Optional: Pair with roasted vegetables, mashed potatoes, or gravy for a complete meal.