In a 2-quart saucepan, combine the fresh blueberries, white sugar, lemon juice, and the bundled lemon thyme. Bring the mixture to a boil over medium-high heat. Once boiling, reduce the heat to medium-low and simmer for 10 minutes. Lower the heat again and simmer for another 10 minutes, stirring often, until the jam is thickened and jammy. Discard the thyme bundle and let the jam cool for at least 10 minutes.

Lay out 4 slices of sourdough bread. Spread a generous layer of the cooled blueberry jam onto each slice. Sprinkle an even mixture of the shredded white cheddar and mozzarella cheeses over the jam. Top with the remaining 4 bread slices.

Heat a large skillet (cast iron works best) over medium heat and melt 2 tablespoons of butter. Place two sandwiches in the skillet and cover with a lid to help the cheese melt. Cook for 2-3 minutes until the bottom is golden brown. Flip and cook for another 2-3 minutes. Repeat the process with the remaining butter and sandwiches.

Remove the sandwiches from the heat and cut them in half. Sprinkle with a touch of flaky sea salt and extra fresh lemon thyme leaves for a professional, aromatic finish.