01 - Combine blueberries with honey and lemon zest (if using) in a small bowl, stirring gently to coat.
02 - Evenly spread softened cream cheese onto one side of each bread slice.
03 - Sprinkle shredded mozzarella evenly on top of cream cheese on half of the bread slices.
04 - Distribute the blueberry mixture evenly over the cheese-coated bread slices.
05 - Place the remaining bread slices, cream cheese side down, over the fillings to form sandwiches.
06 - Spread softened butter evenly on the outer sides of each sandwich.
07 - Heat a nonstick skillet over medium heat. Place sandwiches in the skillet and toast for 3 to 4 minutes per side, pressing gently, until bread is golden and cheese is melted.
08 - Remove sandwiches from the skillet, allow to cool slightly, slice in half, and serve warm.