01 - In a small bowl, gently mash the blueberries with honey (or maple syrup) and lemon zest until slightly broken down but still chunky. Set aside to allow the flavors to meld.
02 - Spread softened butter evenly on one side of each bread slice. If using Parmesan for extra crunch, press the buttered sides into the grated Parmesan to coat thoroughly.
03 - Place two slices of bread buttered-side down. Layer each with a slice of sharp white cheddar, spoon the blueberry filling evenly over the cheese, then top with another slice of cheddar. Cap each sandwich with the remaining bread slices, buttered-side facing outward.
04 - Heat a large skillet or griddle over medium-low heat. Carefully 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 to allow the filling to set. Slice diagonally and serve immediately while warm and crisp.