Preheat a skillet or grill pan over medium heat.
Lightly butter one side of each bread slice.
Place one slice of bread butter-side down in the skillet.
Layer cooked chicken evenly on the bread.
Add avocado slices and drizzle with lemon juice if using.
Season with salt and black pepper.
Place cheese slices on top of the filling.
Cover with the second bread slice, butter-side up.
Cook until golden brown, then flip carefully.
Cook until the cheese is melted and bread is crisp.
Remove from heat, slice, and serve warm.
