Mash the ripe avocado with fresh lemon juice, adding a pinch of salt and pepper to taste. Set aside.

Take two slices of bread and spread mayonnaise or aioli on one side of each slice. On the unbuttered side of one slice, layer the avocado mixture, followed by chicken and cheese.

Heat a skillet over medium heat and add butter. Place the sandwich in the skillet and cook for about 3-4 minutes until golden brown.

Flip the sandwich and add more butter if needed. Toast for another 3-4 minutes until crispy.

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