Preheat the oven to 375°F. Line a rimmed baking sheet with foil or parchment paper.
In a small bowl, combine the butter, garlic, mozzarella, cheddar, Monterey Jack, scallions and salt.
Cut the baguette in half horizontally.
Spread the cheesy garlic butter on the cut sides of the bread. Place the baguette halves on the prepared baking sheet. Bake for 10 to 14 minutes until the bread is hot and the top is lightly browned and melty. Sprinkle with the parsley. and some red pepper flakes if desired. Serve hot.