Preheat oven to 350-degrees F.
Combine garlic and butter in a microwave safe bowl and melt in microwave in 30 second increments until butter is completely melted. Set aside.
Place the bread onto a sheet pan with a large enough sheet of aluminum to cover the loaf of bread halfway.
Cut the bread into 1 inch diamonds. Ensure that you do not cut all the way through the bread to the bottom. It should still be attached at the bottom.
Use a knife or a spoon to label in a little bit of pesto in each slit. I found it was the easiest to just follow the lines with my spoon.
Next, open up each slit and place the mozzarella cheese into each, ensuring all the slits are full and all the cheese is gone.
Next use a pastry brush to brush the top of the loaf with the garlic butter.
Use another piece of aluminum to cover the top exposed area of the bread. Close up tightly and bake for 20 minutes.
After the initial 20 minutes is done, remove the top part of the foil and continue baking the bread until it golden ad crispy on the top. This will take about 5-7 minutes.
Remove from the oven, garnish with diced tomatoes and serve immediately.