Using a sharp bread knife cut the bread going both directions. Do not cut through the bottom crust.
Places slices of cheese between cuts. Mix together butter, onion, and poppy seeds. Pour over bread. Wrap in foil the entire thing in foil and place on a baking sheet. Bake at 350 degrees for 15 minutes. Unwrap. Bake 10 more minutes, or until cheese is melted.