Position a rack in the upper-middle of the oven and heat to 425°F. Place a rimmed baking sheet on the rack to preheat.
Bloom the garlic: Melt 2 tablespoons of the butter in a small skillet over low heat. Stir in the grated garlic and warm just until fragrant, 30–60 seconds. Scrape into a medium bowl and let cool for 1 minute.
Make the spread: Add the remaining 4 tablespoons softened butter and the garlic powder to the bowl; mash to a smooth paste. Fold in the Parmesan, half of the mozzarella, the green onions, 1 tablespoon of the parsley, and the red pepper flakes until evenly combined.
Prep the loaf: Split the bread lengthwise with a serrated knife. Place both halves cut-side down on the hot baking sheet and toast for 3–4 minutes to dry and crisp the surface.
Assemble: Flip the halves cut-side up. Spread the garlic-cheese mixture edge to edge. Sprinkle the remaining mozzarella over the tops.
Bake for 7–9 minutes, until the cheese is fully melted and the edges of the bread are golden.
Broil to finish: Switch the oven to broil and brown the tops for 45–90 seconds, watching closely. Rest 3 minutes, then scatter the remaining 1 tablespoon parsley over the bread.
Slice into 8–12 pieces and serve hot.