Preheat the oven to 350°F.

Press the ground beef evenly into a 9x13-inch baking dish, spreading it all the way to the edges.

Season the meat and sprinkle the minced onion evenly over the top.

Bake for 20 minutes, or until the beef is fully cooked through.

Carefully drain off any excess grease.

If needed, slice the buns in half and place the bottom halves in a baking dish.

Layer the cheese slices over the buns, then add the cooked beef evenly on top.

Place the top halves of the buns over the meat.

Brush the tops with melted butter and sprinkle with sesame seeds, if desired.

Return to the oven and bake for 10-15 minutes, or until the tops are golden brown.