01 - Combine shredded chicken breast with BBQ sauce in a medium bowl, tossing until evenly coated.
02 - Butter one side of each sourdough bread slice. Place buttered side down on preparation surface.
03 - Place one cheddar slice on four bread slices. Top with BBQ chicken mixture, red onion slices, pickled jalapeños, mozzarella slice, and remaining cheddar slice. Complete with second bread slice, buttered side facing outward.
04 - Heat large skillet or griddle over medium heat. Cook sandwiches 3–4 minutes per side, pressing gently with spatula, until golden brown and cheese melts completely.
05 - Remove from heat, cut each sandwich diagonally in half, and serve immediately while hot.