01 - In a small bowl, mash one avocado with lemon juice, mayonnaise, Dijon mustard, salt, and pepper until smooth and creamy.
02 - Spread the prepared avocado mixture evenly on 4 bread slices, ensuring complete coverage to the edges.
03 - Layer shredded chicken, tomato slices, remaining avocado slices, and cheese slices on top of the avocado spread. Sprinkle with cilantro or parsley if desired.
04 - Top with remaining bread slices to form 4 complete sandwiches, pressing gently to adhere layers.
05 - Butter the outside of each sandwich lightly on both sides, ensuring even coating for golden grilling.
06 - Heat a large skillet or griddle over medium heat. Grill sandwiches for 3-4 minutes per side, pressing gently with spatula, until bread is golden brown and cheese is thoroughly melted.
07 - Remove from heat, slice each sandwich diagonally in half, and serve immediately while warm.