Slice the top and bottom off the grapefruit so it sits upright on your board. Using a small, sharp knife, carefully slice off the skin and white pith, cutting as close to the flesh as possible. Keep hold of the skin for later. Slice the grapefruit into segments and pop them into a bowl.
Halve the cucumber. Set one half aside and use in salads or sandwiches. Slice the remaining half lengthways and chop it into matchsticks. Add it to the grapefruit with the bean sprouts.
Trim the roots off the salad onion and very finely slice it, both the green and white parts. Add it to the bowl. Pick the coriander and mint leaves off the stalks and add to the bowl. Toss everything together to mix.
Finely chop the chilli, flicking out the seeds and white bits for less heat. Put the diced chilli in a small bowl. Finely grate in the lime zest, then squeeze in the juice. Add all the tamari and 1 tsp of the Demerara sugar.
Hold the grapefruit skin over the small bowl and press down on any flesh that has been sliced off with it to squeeze out any juice. Whisk the citrus dressing together. Taste and add more sugar if you think it needs it. Add the dressing to the salad ingredients and toss a few times to make sure everything is coated.
Rub 1 tsp oil into the skin of each salmon fillet. Put the fillets, skin-side down, into a non-stick frying pan and set the pan over a low heat. Cook for 5 mins, then turn up to a medium heat and cook for 3 mins. Turn the heat up again, to high, and cook for another 3-4 mins till the skin is crisp and sizzling underneath.
Turn the heat under the pan down to medium and flip the fillets onto one of their sides. Cook for 1-2 mins, then flip over and cook for another 1 min so they're cooked all the way through. Lift out of the pan and set aside for 5 mins to cool.
Peel the skin off the salmon and flake the fillets into the salad. Toss a few times to coat in the dressing, then divide between 2 warm bowls. Break the salmon skin into pieces and add to the bowls just before serving.