Make Ahead:
Combine butter, garlic, parsley and Parmesan cheese as recipe directs. Store in the refrigerator up to 3 days in advance. Soften at room temperature before using.
Day Of:
Preheat oven to 375 degrees F. Cut slices down the length of bread loaf at 1-inch intervals, without cuting all the way through.
Rub slices and top of loaf with cut sides of garlic halves. Mince garlic.
Combine butter, garlic, parsley and Parmesan in a small bowl. Spread butter mixture between bread slices and on top of loaf.
Wrap bread in foil, leaving top partially uncovered. Bake until heated through, about 15 minutes.