01 - Combine shredded chicken breast with BBQ sauce in a medium bowl, tossing until evenly coated throughout.
02 - Layout all sourdough slices and spread softened butter on one side of each piece. Place bread butter-side down on preparation surface.
03 - On four buttered bread slices, layer sharp cheddar, BBQ chicken mixture, red onion, jalapeños, mozzarella slice, and remaining cheddar. Top with second bread slice, buttered side facing outward.
04 - Heat large skillet or griddle over medium temperature. Cook sandwiches 3-4 minutes per side, applying gentle pressure, until bread achieves golden-brown crust and cheese melts completely.
05 - Remove from heat, cut each sandwich diagonally in half, and serve immediately while hot.