Tip the rice into a bowl. Cover with cold water. Whisk for 1-2 mins (you can use a fork) till the water goes cloudy (this is the starch). Drain and rinse under cold water.
Tip the rice into a small pan. Add 150ml boiling water and a pinch of salt. Cover. Bring to the boil. Turn the heat right down and very gently cook for 20-25 mins till the water is absorbed. Take off the heat and set aside, lid on, for 5 mins to finish cooking the rice.
Tip the crab into a bowl. Finely grate in the lime zest. Halve the chilli. Scoop out the seeds and white bits if you want less heat. Finely dice it and add to the crab.
Juice the lime. Whisk the lime juice with 1 tbsp vinegar, 2 tsp sugar and a pinch of salt. Taste and add a little more vinegar, sugar or salt if you think it needs it (it should be a balance of sweet and sour). Add the dressing to the crab and stir to mix. Set aside for the flavours to mingle.
Finely slice the lettuce and toss into a bowl. Trim the roots off the spring onion. Very finely slice it on the diagonal and add it to the lettuce.
Slice the top and bottom off the grapefruit. Slice off the skin and white pith. Slice the segments out of the thin membranes with a small, sharp knife. Add the segments to the bowl, then hold the membranes over the bowl and give them a squeeze to squeeze in any juice. If there's any grapefruit flesh on the skin, give that a squeeze over the bowl too.
Halve the avocados. Remove the stones. Scoop the flesh out of the skins. Slice each half. Set aside.
The rice should be cooked by now. Tip it into a heatproof sieve and rinse under cold water to cool. Shake off any excess water and add the rice to the bowl. Toss everything together, then divide between 2 plates. Top with the avocado and crab. Drizzle over any leftover dressing and serve topped with the coriander leaves.