01 - Combine shredded chicken breast with BBQ sauce in a mixing bowl, tossing until all pieces are evenly coated.
02 - Arrange bread slices and spread a thin layer of softened butter on one side of each slice.
03 - Place 4 bread slices, buttered side down, on a clean cutting board.
04 - Top each bread slice with one fourth of the BBQ chicken, a generous mixture of mozzarella and cheddar cheese (about 1/2 cup total per sandwich), several red onion slices and optional cilantro.
05 - Cover with remaining bread slices, buttered side facing up.
06 - Heat a large skillet or griddle over medium heat. Cook sandwiches 3 to 4 minutes per side, pressing gently with a spatula, until bread is golden brown and cheese is fully melted.
07 - Transfer hot sandwiches to a cutting board, slice diagonally, and serve immediately.