01 - In a small bowl, gently mash the blueberries with honey (or maple syrup) and lemon zest until lightly combined but still chunky. Set aside.
02 - Spread softened butter evenly on one side of each bread slice. If using Parmesan, press the buttered sides into the grated Parmesan to coat them thoroughly.
03 - Place two slices of bread buttered-side down. Layer each with a 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 assembled 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 them rest for 1 minute. Slice each in half and serve immediately while warm and crisp.