Preheat the oven to 425° F.
Mash one stick of salted butter. If you are not using salted butter I recommend adding salt to your butter mixture to enhance the flavors.
Add garlic powder to the butter.
Mince, and using the side of the knife, crush the garlic and add it to the butter and garlic powder.
Finely chop up the fresh parsley and rosemary if you are using fresh rosemary.
Cream the herbs, garlic, and butter together using a rubber spatula.
Slice the sourdough bread horizontally in half.
Evenly spread the garlic herb butter across both sides of the bread.
Add the shredded or sliced mozzarella and a sprinkling of parmesan cheese.
Place the bread on a sheet pan lined with parchment paper and bake for 12-15 minutes or until golden brown around the edges and the butter is bubbling.
Allow the bread to rest for 5 minutes before cutting into it. This will not only allow it to cool down, but also allow al of the flavors to meld together.