Cut the base off the fennel and remove the outer layers. Cut in half lengthways and in half horizontally, then finely slice with the grain. Place in a bowl and toss with the oil, lemon juice and cumin seeds. Leave to marinate for 20 minutes.
Meanwhile, using a sharp knife, remove the peel and pith from the grapefruit. Holding the grapefruit over a bowl to catch the juice, cut down between the membranes and remove the segments. Cut each segment in half, place in the bowl and sprinkle with the salt. Leave to stand for 5 minutes to draw out the sweet juices.
Add the fennel to the grapefruit and mix in the spring onions. Serve topped with the olives.