Score the bread to appear like a checkerboard. About 7 slices across both ways. The cuts should be about ¾ the way down. Be sure not to cut through the bread.
Make the garlic butter. Melt the butter in a small pan over medium heat, then add the garlic, parsley, olive oil, onion powder, Italian seasoning, and red pepper flakes. Simmer for a couple of minutes to allow the flavors to infuse, constantly stirring so the garlic doesn't burn.
Spoon the mixture evenly into the crevices in the bread.
Now stuff the mozzarella and Colby Jack cheese into the crevices. Reserve Parmesan to sprinkle on top of the bread.
Brush the outside of the bread with olive oil, then sprinkle some parmesan on top.
Line a Dutch oven with parchment paper, place the bread inside, and cover with a lid. Bake for 30 minutes with the lid on and 15 minutes with the lid off, or until the cheese is fully melted. Alternatively, you can wrap the bread in foil.
Serve the bread with forks so everyone can pull each piece of bread out. Once all the pieces are pulled out, cut the base of the bread into pieces, kinda like breadsticks.
I like to serve extra olive oil on the side with a sprinkle of salt, pepper, Italian seasoning, and red pepper flakes.
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Notes
Choose a taller loaf - A taller round loaf works best for stuffed garlic bread because it creates deeper pockets for the butter and cheese. This helps the bread hold more filling and keeps everything from spilling out.
Use sturdy bread - A crusty loaf like sourdough holds up well to the butter and melted cheese. Softer sandwich-style breads can become soggy.
Don't cut all the way through the loaf - When scoring the checkerboard pattern, stop about ¾ of the way down. Keeping the base intact helps the bread stay together.
Use a serrated bread knife - A good bread knife makes it much easier to cut clean lines without rushing the loaf.
Stuff the cheese deep into the cuts - Push the cheese down into the crevices instead of just sprinkling it on top so every piece of bread gets that gooey, melty cheese pull.