In a small bowl, combine the melted butter, minced garlic, chopped parsley, Italian seasoning, salt, black pepper, and red pepper flakes if using. Mix well.
Place the loaf of bread on a large cutting board and make diagonal cuts about 1 inch apart, cutting downwards without slicing all the way through. Make similar cuts going the opposite direction to create a diamond pattern.
Carefully widen the cuts slightly and brush the garlic butter mixture generously into each cut, ensuring it's well-distributed.
Gently stuff the shredded mozzarella and cheddar cheese into the cuts of the bread, packing it in as best as you can.
Wrap the bread in aluminum foil, sealing it tightly. This will help the cheese melt evenly.
Place the foil-wrapped bread on a baking tray and bake in the preheated oven for 15-20 minutes.
After 20 minutes, remove the foil and bake for an additional 10-15 minutes, or until the bread is toasted and the cheese is melted and bubbly.
Remove from the oven and let it cool slightly before serving.
Notes
Serve on a wooden cutting board with marinara sauce for dipping.