Using a very sharp knife, cut the fish into thin slices.
Grate the rind from 1 lime into a medium-sized bowl, then squeeze in the juice from both limes. Cut off the base of the grapefruit, then cut around the flesh to remove the rind and pith. Slice into segments and set aside. Add any juice from the grapefruit to the bowl. Add the fish to the lime and grapefruit juices and toss to coat. Leave to marinate for 15 minutes.
Discard the marinade from the fish. Arrange the fish on a serving plate with the grapefruit segments and tomatoes. Scatter over the chilli and mint, season and drizzle with the oil to serve.