Preheat your oven to 350°F (175°C).

In a mixing bowl, combine the cooked chicken, mayo, lemon juice, salt, and pepper.

Layer avocado slices on top of the mixture.

Place the mixture between two slices of bread, adding cheese if desired.

Bake in the preheated oven for 10-15 minutes or until heated through and cheese is melted.

Remove from oven, let cool for 5 minutes, and serve warm.