Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper or foil. Using a serrated knife, slice the loaf in half lengthwise to create two long halves. If you prefer pull-apart slices, alternatively cut the loaf crosswise into 1–1½-inch thick slices without cutting all the way through (leave the base intact) — both methods work; directions below assume halved loaf unless noted.

Make garlic butter

In a medium bowl, combine the softened unsalted butter, finely minced garlic, chopped parsley, salt, and black pepper. Stir until uniformly blended. If the butter is very soft, beat with a fork to incorporate some air for spreadability. Stir in the grated Parmesan now if using. Taste and adjust seasoning (add a pinch more salt if needed).

Assemble

If using halved loaf: Spread the garlic-parsley butter evenly over the cut sides of both halves. Lightly brush the outside crust with the optional olive oil to encourage browning. If using sliced pull-apart method: spread the butter between and on top of the slices so garlic mixture gets into the cuts.

Bake

Place the prepared halves (butter side up) or the sliced loaf on the lined baking sheet. Bake in the preheated oven for 10–14 minutes, until butter is melted and bread is heated through. For a crisp, golden top, switch the oven to broil on high for the last 1–2 minutes — watch closely to avoid burning.

Finish

Remove garlic bread from the oven. If desired, immediately sprinkle the optional red pepper flakes and extra chopped parsley over the hot bread. Let rest 1–2 minutes to firm slightly, then slice (if halved) into serving pieces or serve as pull-apart portions.

Serving

Arrange on a platter and serve warm as a side to pasta, soup, salad, or as an appetizer. Leftovers can be reheated in a 350°F (175°C) oven for 8–10 minutes or refreshed in a toaster oven.