In a medium saucepan over medium heat, combine fresh blueberries, white sugar, lemon juice, and lemon thyme. Stir gently and bring the mixture to a boil. Once boiling, reduce the heat to low and allow it to simmer for about 10 minutes, or until the compote thickens and becomes syrupy.

After simmering, remove the saucepan from heat and let the blueberry compote cool for 10 minutes.

Take half of your artisan bread slices and spread an even layer of the cooled blueberry compote on each slice. Sprinkle shredded white cheddar and mozzarella cheese over the compote.

Place the remaining bread slices on top to form sandwiches. Press them gently together.

Heat a cast-iron skillet over medium heat and add a tablespoon of unsalted butter. Once melted, place the assembled sandwiches in the skillet. Grill each side for about 2-3 minutes until golden brown and crispy.

Once grilled, remove the sandwiches from the skillet and let them cool for a minute. Cut in half and sprinkle lightly with flaky sea salt if desired.