Combine the beef, salt, pepper, and garlic powder in a 9x13-inch (23x33-cm) rimmed baking dish, mixing thoroughly, then pressing into a flat, even layer. Bake for 20 minutes. Drain the liquid and set the cooked beef aside.
Slice the rolls in half lengthwise. Place the bottom half in the same baking dish. Place the cooked beef on the rolls, followed by the onions and cheese. Top with the remaining rolls.
Brush the tops of the rolls with melted butter and sprinkle the sesame seeds on top. Bake for 20 minutes, or until the bread is golden brown and the cheese is melted.