Slice the fish into 1cm strips, place in a bowl and pop in the fridge.
Halve, deseed and chop the chillies into 1cm rounds. Peel and grate or finely chop the garlic, then place in a separate bowl or jam jar. Mix in the lemon juice and 1 teaspoon of sea salt, then pop in the fridge.
Cut the top and bottom off the grapefruit, carefully peel away the skin, then separate them into segments and put these in a bowl, squeezing the juice from a few segments into the bowl.
Reserving the bushy tops, trim and cut the fennel into thin wedges. Pick the mint leaves.
When you're ready to eat, get the fish out of the fridge and combine it with the fennel, grapefruit and most of the mint leaves. Give the marinade in the bowl or jam jar a mix, then pour the juices over the fish mixture. Delicately toss together and leave for 2 minutes 30 seconds.
Serve simply on a big platter, with the remaining mint leaves and the fennel tops sprinkled over, adding a little drizzle of oil and a few grinds of black pepper.