Start by browning the ground beef in a large skillet over medium-high heat, breaking it into small crumbles as it cooks. Once the meat is fully cooked and no longer pink, drain off any excess grease to keep the sauce from becoming too oily.
Transfer the browned beef to your slow cooker along with the diced onions, minced garlic, and chopped bell peppers. In a separate bowl, whisk together the root beer, ketchup, brown sugar, Worcestershire sauce, mustard, salt, pepper, garlic powder, and onion powder until smooth.
Pour this sauce mixture over the beef and vegetables in the slow cooker, then stir everything together gently to combine. Cover the slow cooker and cook on Low for 4 to 6 hours or on High for 2 to 3 hours. The longer cooking time allows the flavors to meld together beautifully.
About 20 minutes before serving, stir in the quick-cooking oatmeal to thicken the sauce. This will help the mixture stick to the buns without being too runny. Spoon the sloppy joe mixture onto slider buns and top with cheese slices or pickles if desired.
Notes
Drain the beef thoroughly to prevent the sauce from becoming greasy. Pat it with paper towels if needed to remove extra fat.
Toast the slider buns lightly in the oven or on a griddle for extra texture. This helps them hold up better against the saucy meat.
For thicker sloppy joes, remove the lid during the last 20 minutes of cooking to let some liquid evaporate. You can also add an extra tablespoon of oatmeal.
Adjust the sweetness by reducing the brown sugar if you prefer a less sweet version. You can also use less root beer and add beef broth instead.
Make it ahead by preparing the sloppy joe mixture the night before and refrigerating it. In the morning, just transfer it to the slow cooker and turn it on.