01 - In a small bowl, gently mash the blueberries with honey or maple syrup and lemon zest until slightly chunky. Set aside to allow flavors to meld.
02 - Spread butter evenly on one side of each bread slice. If using Parmesan, press the buttered sides into the grated Parmesan to coat lightly.
03 - Place two slices of bread buttered-side down. Layer each with one slice of sharp white cheddar, spoon the blueberry mixture evenly over the cheese, then top with the remaining cheddar slices. Cover with the remaining bread slices, buttered-side facing outward.
04 - Heat a large skillet or griddle over medium-low heat. Place the sandwiches in the skillet and cook for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is deeply golden and the cheese is fully melted.
05 - Remove the sandwiches from the skillet and let rest for 1 minute. Slice diagonally and serve immediately while warm and crispy.