01 -
Combine fresh blueberries, white sugar, lemon juice, and the lemon thyme bundle in a small saucepan. Cook over medium heat, stirring occasionally, until blueberries begin to burst and the mixture thickens, approximately 8 minutes. Remove thyme bundle and let filling cool slightly.
02 -
Evenly spread softened butter on one side of each slice of sourdough bread.
03 -
Lay out half the bread slices, buttered side down. Layer each with grated white cheddar and shredded mozzarella. Spoon a generous portion of cooled blueberry filling over the cheese. Top with remaining bread slices, buttered side up.
04 -
Heat a large nonstick skillet or griddle over medium-low heat. Transfer sandwiches to skillet and cook until bread is golden brown and cheeses have melted, about 3 to 4 minutes per side. Work in batches if necessary.
05 -
Remove from skillet and let sandwiches rest briefly. Cut in halves or quarters as desired, sprinkle with flaky sea salt, and serve immediately while hot and crisp.