In a medium saucepan over medium heat, combine fresh blueberries, white sugar, freshly squeezed lemon juice, and lemon thyme. Stir until combined and bring the mixture to a boil. Reduce the heat and let it simmer for about 20 minutes, stirring occasionally, until the compote thickens and is slightly bubbly. Once thickened, set aside to cool while you assemble the sandwich.

Take two thick slices of bread and spread an even layer of the cooled blueberry compote on one side of each slice. Carefully layer the white cheddar and mozzarella cheese on top of the compote, ensuring complete coverage for gooey goodness. Place another slice of bread on top to create a luscious sandwich, pressing down gently to help the layers stick together.

Heat a skillet over medium heat and add a generous amount of butter, allowing it to melt and coat the pan. Once the butter is bubbling, carefully place the assembled Crispy Blueberry Grilled Cheese sandwiches in the skillet. Grill for 2-3 minutes on one side until golden brown and the cheese begins to melt, then flip the sandwiches to grill the other side.

Once both sides are beautifully browned and crisp, remove the sandwiches from the skillet. If desired, sprinkle a touch of flaky sea salt and extra lemon thyme on top for added flavor. Cut each sandwich in half diagonally and serve immediately while still warm for the best experience.