Combine the blueberries, sugar, lemon juice, and lemon thyme in a 2qt saucepan over medium-high heat. Bring to a boil. Reduce the heat to medium-low and let the mixture simmer for about 10 minutes. Reduce the heat to low and simmer for another 10 minutes, or until thickened, stirring often. Remove the lemon thyme. Let cool for 10 minutes.

Lay 4 slices of bread out. Spread the jam onto the bread. Distribute the white cheddar and mozzarella cheeses over top. Close the sandwiches with the remaining 4 slices of bread.

Heat a 12” cast iron skillet over medium heat. Add 2 tablespoons of butter. Let it melt. Place two of the sandwiches into the skillet. Cover the skillet with a lid. Cook for 2-3 minutes until golden. Flip the sandwiches and continue to cook for 2-3 more minutes until the bread is golden and the cheese has melted (lower the heat as needed during the cooking process so the bread doesn’t burn).

Transfer the sandwiches to a cutting board. Add the remaining 2 tablespoons of butter and repeat with the remaining sandwiches.

Cut the sandwiches in half. Top with flaky sea salt and extra lemon thyme leaves.