01 - Spread 1 tablespoon of softened butter evenly on one side of each bread slice. Flip the slices butter-side down.
02 - Spread about 1½ tablespoons of basil pesto on the unbuttered side of two bread slices.
03 - Layer 2 slices of mozzarella cheese and sprinkle 1 tablespoon grated Parmesan over the pesto. Add tomato slices or fresh spinach if using.
04 - Top each sandwich with remaining bread slices, buttered side facing outward.
05 - Heat a non-stick skillet over medium heat. Place sandwiches in the skillet and cook for 3-4 minutes per side, pressing gently with a spatula until golden brown and cheese is melted.
06 - Remove from skillet and let rest for 1 minute. Slice diagonally and serve warm.