Add the blueberries, granulated sugar, lemon juice, and tied lemon thyme sprigs to a medium saucepan.

Place the saucepan over medium-high heat and bring the blueberry mixture to a gentle boil.

Reduce the heat to medium-low and simmer for approximately 10 minutes, stirring occasionally.

Reduce the heat to low and continue simmering for another 10 minutes, or until the berries have broken down and the mixture becomes thick and jammy. Stir frequently to prevent sticking.

Remove and discard the lemon thyme bundle.

Transfer the blueberry mixture away from the heat and allow it to cool for approximately 10 minutes. It will continue to thicken as it cools.

Arrange 4 slices of sourdough bread on a clean work surface.

Spread an even layer of blueberry jam over each slice, leaving a small border around the edges to reduce leaking during cooking.

Divide the shredded white cheddar and mozzarella evenly among the four sandwiches.

Top each sandwich with one of the remaining sourdough slices.

Heat a large cast-iron skillet over medium heat. Add 2 tablespoons of butter and allow it to melt.

Place two Blueberry White Cheddar Grilled Cheese sandwiches into the skillet.

Cover the skillet and cook for 2–3 minutes, or until the bottom of each sandwich is crisp and golden.

Carefully flip the sandwiches. Cover again and cook for another 2–3 minutes, until both sides are golden and the cheese is completely melted.

Lower the heat if the bread begins browning faster than the cheese melts.

Transfer the finished sandwiches to a cutting board.

Add the remaining 2 tablespoons butter to the skillet and repeat with the remaining sandwiches.

Cut each sandwich in half and finish with a small pinch of flaky sea salt and fresh lemon thyme leaves.