With a sharp bread knife, cut bread in a grid-like (cross-hatch) pattern, taking care NOT to cut through the bottom crust.
Insert cheese slices between each deep cut (about 2 dozen slices).
Place melted butter, scallions, and poppy seeds in a small bowl. Stir to mix well. Make a blanket of aluminum foil large enough to totally encompass the bread. Place bread in the center of the foil. Drizzle butter mixture over bread and cheese.
Wrap foil tightly around bread and place it on a baking sheet. Bake for 15 minutes; open foil and bake 10 minutes more, until cheese is melted.
To serve, place unwrapped bread in a napkin-lined bread basket. Guests serve themselves by ripping pieces of cheese bread from the loaf.
SUBSTITUTE: You can substitute a flavored artisan bread for the sour dough.