Preheat the oven to 350°F.
In a medium bowl, stir together the butter, garlic, and parsley until combined. Season to taste with salt and pepper.
Cut the bread lengthwise into two planks, and slather the cut sides with the butter, spreading evenly.
Sandwich the bread together, buttered sides touching, then wrap the loaf in foil.
Place the foil-wrapped loaf on a rimmed baking sheet and transfer to the oven.
Bake for 10 minutes, then unwrap the loaf and arrange the two halves side by side on top of the foil, buttered-side up.
Sprinkle with the Parmesan, dividing evenly, and top with some red pepper flakes to taste.
Preheat the broiler and arrange an oven rack in the top position.
Return the bread (still on the pan) to the top rack and broil until deep golden brown, watching carefully so the bread doesn’t burn, 2 to 3 minutes.
Remove from the oven, transfer to a cutting board, and cut each half crosswise into thick batons.
Serve warm.
