For the avocado cheese sandwich, cut the avocado in half, pit it and scoop out the flesh with a spoon.
Mash the flesh with a fork and mix with lemon juice and mayonnaise.
Spread the avocado cream on half of the bread slices, top each with lettuce leaves and a slice of cheese, and sprinkle with cress and chili flakes to taste.
Cover with the remaining slices of bread and cut the avocado cheese sandwich diagonally.