In a small saucepan, combine blueberries and sugar over medium heat. Cook for 5-7 minutes until you achieve a jam-like consistency. Let this flavorful mixture cool down for a few moments.

Spread a generous layer of cream cheese or brie on one side of each slice of bread. For two slices, add mozzarella on top of the cheese, then spoon the cooled blueberry mixture over it. Place the remaining slices on top, cream cheese side down.

Heat a skillet over medium-low heat while generously buttering the outside of the sandwiches. Place them in the skillet and cook for 3-4 minutes per side until golden and the cheese has melted.

Allow the sandwiches to rest for 1-2 minutes before slicing them diagonally. Finish by sprinkling with flaky sea salt.