Remove any coarse outer lettuce leaves, separate and wash the remainder. Remove any coarse stems and pat the leaves dry. Place in a salad bowl and add the peeled and sliced cucumber and the washed sprigs of watercress.
Peel the avocado, halve it and remove the centre stone. Slice the flesh and toss immediately in a few tablespoons of the oil and vinegar dressing. When ready to serve, add to the salad and toss with the remaining dressing. Fry the chopped bacon in a dry pan until the fat runs and the pieces are crisp. Sprinkle over the salad and serve at once.