To make the vinaigrette, grate the lime and lemon zest into a bowl - I find using a Microplane is the easiest way to do this but you could use the finest side of a grater. Squeeze the lime and lemon juice into the same bowl as the zest, making sure none of the pips sneak in. Remove the tough outer skins of the spring onions, chop finely and add to the bowl.
Roll the chilli between your hands to loosen the seeds. Cut off the top and slice the chilli in half length-wise, scrape out and discard the seeds. Slice length-wise into fine strips, chop finely and add to the bowl. Add the sesame oil, olive oil and soy sauce and mix thoroughly.
Now take a handful of coriander leaves (try to avoid the stems) and chop as finely as you can. You need enough to make a good, heaped tablespoon. Add to the bowl and stir. Add a good grind of pepper, pinch of salt and the sugar, stir and that's it, the vinaigrette's ready. You can make this 2 or 3 days ahead if you like.
Stretch a couple of sheets of clingfilm taut over a large platter to place the slices of salmon on - this way they won't stick to the surface of the plate and break when you come to dress them. To get neat, even slices the fish needs to be really cold, and you'll need a sharp, unserrated knife. Slice it as soon as it comes out of the fridge or, even better, leave it in the freezer for about 10 minutes before you slice it. Slice the salmon fillet across, aiming for no thicker than 5mm, to give 6 or 7 slices per person. Arrange each slice on the clingfilm.
If you're not eating for a while, put it in the fridge - just remember to remove it about half an hour before your guests arrive so that it has time to get back up to room temperature so that you can appreciate the full flavour of the fish.
About five minutes before you're ready to eat, choose your salad leaves. I like using micro salad leaves such as shiso cress, green broccoli, mizuna, radish and purple basil but pea shoots, oak leaves and lollo rosso are also good. Use whatever you prefer. It's good to get two colours, purple and green for contrast. Some supermarkets sell mixed variety packs.
Take a strip of salmon and dip it into the vinaigrette, curl into a rosette - you don't need to be too fussy about this, just make sure it is roughly a round shape, and start in the centre of the plate. Repeat with the other slices, placing them around this centre rosette, aiming for around 6 or 7 pieces on each plate.
Now scatter the salad leaves over the fish, dipping a few individual leaves into the dressing as you go. Finish by spooning a little more vinaigrette over and around the salmon.
Serve immediately - the dressing 'cooks' the fish so don't dress more than five minutes before eating as it will turn the fish to mush. This would make an invigorating starter before a roast beef or lamb main course.
Ceviche Ceviche is basically raw fish 'cooked' in citrus juice. It relies on spankingly fresh fish, so make sure you get to a good fishmonger early in the day. If you prefer, use sea bass, sea trout, rainbow trout or brill - all good alternatives to salmon.