In a medium mixing bowl, whisk together the honey, apple cider vinegar, olive oil, smoked paprika, 1 tablespoon salt, garlic, cayenne, and white part of the scallions (reserve the green parts for serving).
Spray insert of a 5-quart or larger slow cooker with cooking spray. Place pork loin in the insert, then pour the honey mixture over the top. Cover and slow cook for 6 hours on low or 4 hours on high, until the pork registers 165 degrees F on a thermometer and is no longer pink in the center. With tongs, gently remove the pork from the slow cooker and transfer it to a large plate, reserving the pork juices in the slow cooker. With two forks, shred the pork. Set aside.
Transfer the pork juices remaining in the slow cooker to a medium saucepan over medium-high heat. In a small bowl, whisk together the cornstarch and water, then add to the cooking liquid. Cook, whisking constantly, until the sauce bubbles and thickens, 5 to 10 minutes. Whisk in remaining 1 teaspoon salt. Return shredded pork to the slow cooker and pour the thickened sauce over the top. Toss gently to combine, then turn the slow cooker to low to keep the pork warm.
Place rack in the center of oven and preheat oven to 200 degrees F. .
To serve: Serve on toasted onion roll or other roll of choice.. Garnish with reserved green onion tops and serve warm.