Heat a large skillet over medium-high heat. Add the ground beef, green bell pepper, and onion.
Cook for 6 to 8 minutes, breaking up the beef as it cooks, until the meat is browned and the vegetables are tender. Drain any excess grease.
Stir in the ketchup, celery seed, dry mustard, vinegar, granulated sugar, salt, and black pepper. Simmer for 15 minutes, stirring occasionally.
While the filling simmers, whisk together the melted butter, brown sugar, Worcestershire sauce, Dijon mustard, sesame seeds, and onion powder in a small bowl.
Keeping the rolls connected, slice them horizontally through the center to create a top and bottom layer.
Place the bottom half of the rolls in a 9 x 13-inch baking dish.
Spread the sloppy joe mixture evenly over the rolls.
Arrange the cheese slices over the meat mixture and sprinkle the French fried onions on top.
Place the top half of the rolls over the filling.
Brush the glaze evenly over the tops of the rolls.
Bake for 10 to 15 minutes, or until the cheese is melted and the tops are lightly golden brown.
Slice into individual sliders and serve warm.
Christina's Notes
Ground turkey, ground chicken, or ground pork can be substituted for the ground beef.For extra vegetables, add finely chopped mushrooms, shredded carrots, or spinach to the sloppy joe mixture while browning the meat.Try adding a splash of barbecue sauce, Worcestershire sauce, or a pinch of garlic powder for a different flavor profile.Brioche slider buns work beautifully in place of Hawaiian rolls if you prefer a less sweet sandwich.For a shortcut version, use canned sloppy joe sauce and simply brown the beef before assembling the sliders.Store leftover sliders covered in the refrigerator for up to 3 days.Reheat covered in a 325 degree F oven for 10 to 15 minutes or microwave individual sliders until heated through.The sloppy joe filling can be made up to 2 days ahead and refrigerated until ready to assemble.Freeze the cooled sloppy joe filling for up to 3 months. Thaw overnight in the refrigerator before using.