Slice loaf lengthwise without cutting completely through the bread. Spread half of the butter on inside surfaces; season with salt and pepper. Coat the outside of the bread with remaining butter; season with salt and pepper.
Place loaf cut-side down on the grill; toast until browned, checking often, 3 to 5 minutes. Close loaf using tongs. Grill surface until crispy, checking often, 3 to 5 minutes per side.
Transfer loaf to a cutting board using tongs; slice into 10 diagonal slices.