In a medium saucepan over medium heat, combine fresh or frozen blueberries with a sprinkle of sugar. Mash the berries gently with a fork for 5–7 minutes until thick, resembling chunky jam. Remove from heat and set aside to cool.

Spread a generous layer of cream cheese or brie on one side of each slice of bread to create a creamy base.

On two slices with the creamy side facing up, layer shredded mozzarella or fontina over the cream cheese. Spoon the cooled blueberry mixture on top, then top with remaining slices, creamy side down.

Preheat a skillet over medium-low heat. Butter the outer sides of each sandwich liberally.

Place sandwiches in the skillet and cook for 3–4 minutes until golden brown. Flip and cook for another 3–4 minutes until cheese melts and bread is crispy.

Let the sandwiches rest for 1–2 minutes, slice diagonally, and sprinkle with flaky sea salt before serving.