In a medium saucepan over medium heat, combine fresh or frozen blueberries and a sprinkle of sugar. Cook for 5-7 minutes, stirring occasionally, until the blueberries burst and the mixture thickens into a jammy consistency. Remove from heat and let cool slightly.

Spread cream cheese or brie generously on one side of each slice of bread. On two slices, layer shredded mozzarella or fontina cheese, then spoon the jammy blueberry mixture on top. Place the other slice on top to create sandwiches.

Heat a skillet over medium-low heat, add butter and let it coat the pan. Place the assembled sandwiches in the skillet, butter side down, and cook for 3-4 minutes until golden brown.

Flip the sandwiches carefully using a spatula, butter the top side while cooking, and let cook for another 3-4 minutes until golden brown and cheese is fully melted.

Remove sandwiches from skillet and let rest for 1-2 minutes before slicing. Optionally, sprinkle with flaky sea salt and serve warm.