01 - Combine shredded chicken breast with BBQ sauce in a medium bowl, tossing until evenly coated.
02 - Layout sourdough slices and butter one side of each. Place buttered side down on preparation surface.
03 - Place cheddar slice on 4 bread slices. Top with BBQ chicken mixture, red onion slices, jalapeños, mozzarella slice, and second cheddar slice. Complete with remaining bread, buttered side facing outward.
04 - Heat large skillet or griddle over medium heat. Cook sandwiches 3-4 minutes per side, applying gentle pressure, until golden brown and cheese is fully melted.
05 - Cut each sandwich diagonally in half and serve while hot for optimal texture and flavor.