01 - In a small bowl, gently mash the blueberries with honey or maple syrup and lemon zest until lightly combined. Set aside to allow 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 for an extra-crispy crust.
03 - With the buttered side facing outward, layer two slices of bread with cheddar cheese. Spoon the blueberry mixture over the cheese, then top with the remaining cheese slices. Close each sandwich with the remaining bread, buttered side facing out.
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 gooey.