Clean, wash and drain the salad. Cut into bite-sized pieces. Peel, halve and stone the avocado. Cut the flesh into slices. Clean, wash, quarter and seed the tomato and then cut into small cubes.
Peel shallots and cut into thin rings. Cut cress leaves from the stems. Mix oil and vinegar. Add tomato cubes and shallots, season with salt and pepper. Mix salad, avocado and tomato vinaigrette