1. In a 2-quart saucepan over medium-high heat, combine the blueberries, sugar, lemon juice, and bundled lemon thyme. Bring to a boil, then reduce heat to medium-low and simmer for about 10 minutes.

2. Lower the heat to low and continue to simmer for another 10 minutes, stirring often, until the mixture thickens into a jam consistency. Remove and discard the thyme bundle and let the jam cool for about 10 minutes.

3. Lay out 4 slices of sourdough bread and spread the blueberry jam evenly over each slice. Sprinkle white cheddar and mozzarella over the jam, then top with the remaining bread slices to form sandwiches.

4. Heat a 12-inch skillet over medium heat and melt 2 tablespoons of butter. Place two sandwiches in the skillet and cover with a lid. Cook for 2-3 minutes until golden brown on the bottom.

5. Flip the sandwiches and cook for another 2-3 minutes until the bread is crisp and the cheese is melted. Adjust heat as needed to prevent burning.

6. Transfer to a cutting board. Melt the remaining 2 tablespoons of butter in the skillet and repeat with the remaining sandwiches.

7. Cut each sandwich in half, sprinkle with flaky sea salt and extra fresh thyme if desired, and serve warm.