Warm a large skillet over medium heat.

In a small bowl, blend the softened butter with parsley, Pecorino Romano, and garlic until uniformly combined.

Spread a thin layer of the garlic-herb butter over one side of each bread slice, then flip the slices butter-side down.

Arrange provolone and mozzarella evenly over two of the unbuttered sides, cover with the remaining bread, ensuring the buttered surfaces face outward.

Lay the sandwiches in the skillet and toast the first side until richly golden, about 3–4 minutes; turn gently.

Cook the second side until the bread is crisp, deeply colored, and the cheeses have melted, another 3–4 minutes.

Transfer to a board, halve each sandwich, and serve immediately.