Halve the grapefruit, loosen the segments with a serrated grapefruit knife and place in a bowl.
Scrape out the grapefruit shells to remove all the membrane, then set aside.
Add any residual juice to the segments.
Remove the orange segments in the same way, but do not bother to clean the orange shells -just squeeze the juice from them and add to the orange and grapefruit segments with the sherry.
Pile into the grapefruit shells.
Garnish each with a mint sprig and serve chilled.