Heat the oven to 375°F.
Line a large baking sheet with parchment paper.
Slice the bread lengthwise to create two long halves.
Place both bread halves cut side up on the prepared baking sheet.
Add the softened butter, mayonnaise, minced garlic, chopped parsley, kosher salt, black pepper, Italian seasoning, and crushed red pepper flakes to a bowl.
Stir until the mixture looks smooth and speckled.
Spread the garlic butter mixture evenly over both bread halves.
Push the butter mixture all the way to the edges.
Sprinkle the mozzarella, cheddar, and Parmesan evenly over the buttered bread.
Bake for 10 to 12 minutes, until the cheese melts.
Switch the oven to broil.
Broil for 1 to 2 minutes, watching closely, until the top looks bubbly and lightly golden.
Remove the bread from the oven.
Sprinkle extra fresh parsley over the top.
Let the bread rest for 3 minutes before slicing.
Use a serrated knife to slice the bread into thick pieces.
Serve warm.