In a medium saucepan, combine fresh or frozen blueberries with a sprinkle of sugar over medium heat. Allow to simmer for 5–7 minutes until thick and jam-like.

Spread a generous layer of cream cheese or brie on one side of each slice of bread. Layer shredded mozzarella or fontina on top, then spoon the warm blueberry mixture over the cheese before adding the second slice of bread on top.

Preheat a skillet over medium-low heat, add butter, and once melted, add the sandwiches. Grill each side for 3–4 minutes until golden brown and the cheese is melted.

Remove the sandwiches from heat, let them rest for a minute, then slice diagonally and sprinkle flaky sea salt on top before serving.