Slice bread into ¾” slices, leaving just the bottom of the crust uncut to hold the loaf together. Move sliced bread to a piece of aluminum foil slightly longer than the loaf. (Don’t worry if you slice the bread all the way through; the foil will fix it.)
Stir together the melted butter and pesto. With a basting brush, spread the butter mixture over the top and inside the slices of the loaf.
Cut the cheese slices into pieces (fourths or smaller depending on the size of your cheese and bread), and slide a piece of cheese into each cut of the loaf.
Crimp the foil to surround the bottom edge of the bread to hold it together and catch any delicious drips. Place directly on rack in preheated oven and bake for 15 minutes, or until the top is golden brown and the cheese is melted. Serve warm.