Preheat the oven to 350°F. Spray a 9x13-inch baking dish with nonstick cooking spray.
In a large skillet over medium-high heat, cook the ground beef and diced onion for 5-7 minutes, breaking the beef into small crumbles, until the beef is browned and the onion is tender. Drain any excess grease.
Stir in the ketchup, brown sugar, garlic powder, salt, yellow mustard, Worcestershire sauce, and water. Bring the mixture to a gentle simmer and cook for 4-5 minutes, stirring occasionally, until the sauce thickens to a sloppy joe consistency.
Slice the Hawaiian rolls in half horizontally, keeping the rolls connected if possible. Place the bottom half of the rolls in the prepared baking dish.
Spread the sloppy joe mixture evenly over the rolls. Sprinkle the shredded cheddar cheese over the meat, then place the top half of the rolls on top.
In a small bowl, stir together the melted butter and garlic salt. Brush the mixture over the tops of the rolls and sprinkle with the sesame seeds.
Cover the baking dish with aluminum foil and bake for 15-20 minutes, until the cheese is melted and the sliders are heated through.
Remove the foil and bake for an additional 2-5 minutes, until the tops of the rolls are lightly golden.
Garnish with chopped fresh cilantro, if desired. Let the sliders cool for 5 minutes before slicing and serving.
Notes
These sliders are best served fresh, but you can refrigerate the leftovers in an airtight container for up to 3-4 days.