In a medium bowl, mash a ripe avocado with a splash of lemon juice, adding a pinch of salt and pepper to taste. Use a fork until the mixture is creamy yet slightly chunky.

Spread a thin layer of mayonnaise on one side of each slice of bread. On the unbuttered side of one slice, layer the avocado spread, followed by cooked shredded chicken and a slice of cheese.

Heat a skillet over medium heat and melt a tablespoon of butter. Place the sandwich in the skillet, pressing down gently with a spatula. Cook for 3-4 minutes until the bread is golden brown and the cheese begins to melt.

Carefully flip the sandwich over and cook for another 3-4 minutes until the second side is golden brown and cheese is fully melted.

Remove the sandwich from the skillet, let it rest for a minute, then slice in half and serve warm.