Prepare the Spread: In a small bowl, mix the softened butter with garlic powder, onion powder, oregano, and black pepper until well combined.
Prepare the Bread: Lay out the 8 slices of bread on a clean surface. Spread mayonnaise and mustard on one side of each slice if using.
Assemble the Melts: On the unspread side of 4 slices, layer 2 slices of turkey, followed by 1 slice of cheese. Top with the other 4 slices, placing the spread side facing outward.
Coat the Outside: Generously spread the garlic-herb butter mixture on the outside of each sandwich, ensuring all edges are covered.
Cook the Melts: Heat a large skillet or griddle over medium heat. Place the sandwiches in the skillet and cook for about 3-4 minutes on each side until golden brown.
Serve: Once cooked, remove from heat, let sit for a minute, and cut each sandwich into quarters or halves.