Peel the avocados, remove the pits and slice into thick half rings. Drop the rings into a bowl containing water and lemon juice- this will prevent them from turning black. Cut the pomegranate in half and crush it over a bowl and collect the seeds. Wash the grapes and mix them with pomegranate seeds. Drain the avocado pieces and add them to the fruit. To make dressing, put all ingredients into a glass jar, screw the lid on firmly and shake vigorously for 2 minutes. Pour over the avocado and fruit. Toss well before serving.