Combine the onion, brown sugar, pepper, salt, chili powder, paprika, oregano, garlic, ketchup, water, white vinegar, canola oil, Worcestershire sauce, and liquid smoke in a slow cooker; stir. Lay the beef pieces into the sauce. Cook on LOW for 10 hours.
Remove the beef from the sauce, keeping the sauce in the slow cooker, and shred with two forks. Remove 1/4 cup of the sauce from the slow cooker and pour into a bowl. Whisk the cornstarch into the sauce; stir the mixture back into the slow cooker. Add the shredded beef; stir. Serve hot on the whole-wheat buns.