In a medium resealable plastic bag, combine the flour, garlic powder, salt, and pepper. Add the pork. Seal the bag and shake to evenly coat the cubes. Refrigerate for at least 15 minutes.
Preheat a medium nonstick saucepan over medium-high heat until it is hot enough for a spritz of water to sizzle on it. Add the oil. Scatter the pork cubes into the pan. Cook, turning as needed, for about 5 minutes, or until pork is browned on all sides. Reduce the heat to medium.
Add the orange juice, vinegar, and smoke flavoring. When the mixture comes to a boil, reduce the heat to low so the mixture simmers. Cover the pan. Cook, stirring occasionally, for 1 hour, or until the pork is very tender. With a wooden spoon, shred the pork pieces and mix in the barbecue sauce.
If the onion is desired, 5 minutes before serving, coat a small nonstick skillet with olive oil spray. Set over medium heat. Add the onion. Cook, stirring occasionally, for about 5 minutes, or until tender.
Meanwhile, place the bun halves, cut sides down, in a medium nonstick skillet set over medium heat. Cook for 3 to 5 minutes, or until toasted. Place the bun bottoms on serving plates. Spoon half of the pork mixture onto each bun bottom. Top with onion and additional barbecue sauce if desired. Cover with the bun tops. Serve your barbeque pork sandwich immediately.
Source: Devin Alexander from the Biggest Loser Cookbook