Cut the bread lengthwise and widthwise without cutting through the bottom crust. This can be a little tricky going the second way but the bread is very forgiving.
Place on a foil-lined baking sheet. Insert cheese slices between cuts. Combine butter, onion, and poppy seeds. Drizzle over bread. Wrap in foil; place on a baking sheet. Bake at 350 degrees for 15 minutes.