In a large skillet, brown the ground beef over medium-high heat until no longer pink. Drain the excess fat and transfer the beef to a slow cooker.
Add the chopped onion, red bell pepper, and minced garlic to the slow cooker and stir to combine.
In a separate bowl, whisk together the tomato paste, ketchup, barbecue sauce, brown sugar, Worcestershire sauce, yellow mustard, smoked paprika, salt, and black pepper. Pour the sauce over the beef and vegetables in the slow cooker, and stir to coat everything evenly.
Cover the slow cooker and cook on low for 4-6 hours, or on high for 2-3 hours, stirring occasionally.
Once the beef is cooked through and the vegetables are tender, use a spoon or spatula to break up any large chunks of beef.
Toast the hamburger buns and spoon the sloppy joe mixture onto each bun. Serve hot and enjoy!