Preheat oven to 350°F (175°C) and spray a baking dish with nonstick cooking spray.

In a bowl, combine ground beef with onion powder, salt, and pepper. Form into 12 small patties.

Cook patties in a skillet over medium-high heat for 3-4 minutes on each side until browned; add cheese slices during the last minute.

Slice Hawaiian rolls in half; place the bottom halves in the baking dish. Add patties, ketchup, and mustard before topping with the other half of the rolls.

Brush melted butter over the tops and sprinkle with poppy seeds. Bake for about 15-20 minutes until golden brown.