In a medium saucepan, combine fresh blueberries, white sugar, lemon juice, and lemon thyme. Cook over medium heat until the blueberries begin to burst and release their juices, about 5-7 minutes. Stir occasionally to prevent sticking and ensure even cooking.
Lay out four slices of sourdough bread on a clean surface. Spread a generous amount of the blueberry filling on each slice, followed by an even distribution of grated white cheddar and shredded mozzarella cheese.
Place another slice of sourdough on top of each assembled sandwich to create a classic grilled cheese shape. Generously spread butter on the outer sides of each sandwich, ensuring even coverage for that crispy finish.
Heat a skillet over medium heat and carefully place the sandwiches in the pan once hot. Cook each side for about 3-5 minutes or until golden brown and crispy, flipping gently with a spatula.
Once grilled to perfection, remove from heat and let cool slightly before slicing in half. Sprinkle with flaky sea salt to enhance all those incredible flavors.
Serve your Crispy Blueberry Grilled Cheese warm with extra blueberry filling on the side if desired. Enjoy this innovative take on a classic favorite!