Combine the fresh blueberries, white sugar, lemon juice, and bundled lemon thyme in a 2-quart saucepan over medium-high heat.

Bring the mixture to a boil, then reduce the heat to medium-low and simmer for about 10 minutes.

Lower the heat to low and simmer for another 10 minutes, stirring often, until the jam is thickened.

Remove and discard the lemon thyme bundle.

Allow the jam to cool for 10 minutes before using.

Lay out 4 slices of bread.

Spread the blueberry lemon thyme jam from Step 1 evenly onto each slice.

Sprinkle the white cheddar and mozzarella cheeses over the jam.

Top with the remaining 4 bread slices to form sandwiches.

Heat a 12-inch cast iron skillet over medium heat and add 2 tablespoons of butter, letting it melt.

Place two assembled sandwiches from Step 2 in the skillet and cover with a lid.

Cook for 2-3 minutes until golden on the bottom, then flip and cook for another 2-3 minutes, or until the bread is golden and the cheese is melted.

Adjust the heat as needed to prevent burning.

Transfer the cooked sandwiches to a cutting board, add the remaining 2 tablespoons of butter, and repeat the process with the remaining two sandwiches.

Cut each sandwich in half.

Top with a sprinkle of flaky sea salt and extra lemon thyme leaves if desired, for an aromatic finish.