01 - Toss shredded chicken breast with BBQ sauce in a medium bowl until evenly coated throughout.
02 - Lay out all sourdough slices. Spread softened butter on one side of each slice. Place bread butter-side down on preparation surface.
03 - Place one cheddar slice on four buttered bread pieces. Top with BBQ chicken mixture, red onion slices, jalapeños, mozzarella slice, and remaining cheddar. 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, applying gentle pressure with spatula, until bread turns golden brown and cheese melts completely.
05 - Remove from heat, cut each sandwich diagonally in half, and serve immediately while hot.