In a small saucepan over medium heat, combine fresh or frozen blueberries with sugar. Cook for about 5-7 minutes, stirring occasionally, until the blueberries soften and the mixture turns jammy.

Spread a generous layer of cream cheese or brie on one side of each slice of sourdough or white bread. On two of the slices, add shredded mozzarella or fontina cheese, followed by the blueberry mixture. Top with the remaining slices of bread, cream cheese side down.

Heat a skillet over medium-low heat and spread a thin layer of butter on the outside of each sandwich.

Carefully place the sandwiches in the skillet, cooking for about 3-4 minutes per side, gently pressing with a spatula.

Remove the sandwiches from the skillet and let them rest for 1-2 minutes, optionally sprinkling flaky sea salt before serving.