01 - Spread 1 tablespoon of softened butter evenly on one side of each bread slice.
02 - Flip the bread slices and spread approximately 1½ tablespoons of basil pesto on the unbuttered side of two slices.
03 - Layer mozzarella cheese (2 slices per sandwich) over the pesto-coated side, then sprinkle 1 tablespoon of grated Parmesan cheese on top. Add tomato slices or fresh spinach if desired.
04 - Top each assembled sandwich with the remaining bread slices, ensuring the buttered side faces 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 bread turns golden brown and cheese melts completely.
06 - Remove sandwiches from skillet and let rest for 1 minute. Slice diagonally and serve warm.