Mix the garlic butter. In a bowl, combine the softened butter, minced garlic, salt, dried parsley, ground pepper, and red pepper flakes (if using). Stir until smooth and well combined.
Slice the bread. Using a serrated knife, slice the French bread into individual pieces of even thickness and set aside.
Spread the butter. Spread the garlic butter evenly over one side of each slice, going all the way to the edges to prevent dry spots when baking.
Freeze the slices. Arrange the slices in a single layer on a baking sheet and freeze until solid, about 1-2 hours.
Transfer to storage. Move the frozen slices to a labeled freezer-safe bag and store in the freezer until ready to use.
Preheat the oven. When ready to bake, preheat the oven to 375°F and line a baking sheet with parchment paper.
Bake from frozen. Arrange the frozen slices on the prepared baking sheet and bake for 8-10 minutes, or until hot and golden brown around the edges. No need to thaw first.