01 - In a small bowl, gently mash the blueberries with honey (or maple syrup) and lemon zest until slightly chunky. Set aside to allow the flavors to meld.
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 - With the buttered side facing outward, layer two slices of bread with cheddar cheese. Spoon the blueberry mixture evenly over the cheese, then top with the remaining cheese slices. Cover each sandwich with the remaining bread slices, buttered side facing out.
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 to set. Slice each sandwich in half and serve immediately while warm and gooey.