Brown the meat on the stovetop. Heat the oil in a large frying pan over medium-high heat until shimmering. Add the ground beef and use a stiff spatula or wooden spoon to break the meat into small pieces. Cook, stirring occasionally, until browned and no longer pink, 8 to 10 minutes.
Add the ingredients to the slow cooker. Transfer the browned ground beef to a 4-quart or larger slow cooker. Add the onions, bell pepper, tomato sauce, ketchup, brown sugar, Worcestershire, mustard, garlic powder, salt, and pepper. Stir until well-combined, making sure to break up the beef.
Slow cook the sloppy joes. Cover and cook on the LOW setting low for 4 to 8 hours, or on HIGH setting for 2 to 3 hours.
Assemble the sloppy joes. Stir the mixture again, then divide the mixture between the hamburger buns and serve immediately.
