Cut the avocados all the way around the stone. Twist the two halves apart. Remove the stone, scoop the flesh out of the skin and cut into thin slices.
Slice the chicken and mozzarella thinly.
Brush the bread with the pesto. Divide the avocado, chicken breast and mozzarella over 4 slices. Cover with the other 4 slices (pesto side down).
Heat the butter or olive oil in a large, non-stick frying pan over a medium heat. Fry the sandwiches for approx. 8 until golden brown. Turn them halfway.
Prefer not to use meat? Replace the chicken breast with vegetarian pre-sliced chorizo. For a spicy alternative!
