Preheat the oven to 350 degrees F (175 degrees C).

Slice the tops off of garlic heads, so the tip of each clove is exposed. Place garlic heads, cut-sides up, on a baking sheet; drizzle tops with olive oil.

Bake in the preheated oven until garlic is soft, about 30 minutes.

Set the oven to broil.

Slice bread loaf in half horizontally; place cut-side up on a baking sheet.

Squeeze garlic cloves from their skins into a medium bowl. Stir in butter, Parmesan cheese, and parsley until well blended. Spread onto the cut sides of bread.

Broil bread until toasted, about 5 minutes.