01 - Mash one avocado with lemon juice, mayonnaise, Dijon mustard, salt, and pepper in a small bowl until smooth and creamy.
02 - Spread the prepared avocado mixture evenly across 4 bread slices, ensuring full coverage to the edges.
03 - Arrange shredded chicken, tomato slices, remaining fresh avocado slices, and cheese slices over the avocado spread. Sprinkle with chopped cilantro or parsley if desired.
04 - Place the remaining 4 bread slices on top to close the sandwiches, pressing gently to adhere.
05 - Generously butter the outside of each sandwich on both sides, ensuring even coverage for optimal golden crisping.
06 - Heat a large skillet or griddle over medium heat. Grill sandwiches for 3-4 minutes per side, pressing gently with a spatula, until bread turns golden brown and cheese melts completely.
07 - Remove sandwiches from heat, slice each diagonally in half, and serve immediately while warm and crispy.