I love rich, oily fish such as mackerel, sardines and anchovies. These types of fish have a lot more flavour than most white-fleshed fish, and I like to serve them with something acidic, which balances the richness. For this grilled mackerel dish, I make a cucumber salad that's seasoned with fish sauce, fresh lime juice and chillies.
Butterflying the fish so the flesh is intact, minus the bones and heads is harder to describe than it is to do. If you have a good fishmonger who is willing to do some extra work (usually for a price), ask them to remove the head and bones while leaving the fish connected at the back.
For the salad, use the small, slender Asian cucumbers which have thin skin, small, tender seeds, and firm, not-too-watery flesh. If you don't have a spiraliser-type of machine, use a julienne peeler - it looks like a regular vegetable peeler, but the cutting edge is serrated, not smooth, so it cuts the vegetable into slender strands. Or, you could cut them into strands with a sharp knife.
Make the cucumber spaghetti. Rinse the cucumbers then cut them (skin, seeds and flesh) into long, thin strands that resemble spaghetti.
Sprinkle about 15 grams (1 tbsp) of salt over the cucumbers and mix well, then leave them in a colander to drain for about 15 minutes. Rinse the strands with cool, running water, squeezing and massaging to remove excess salt. Try a few of the strands, they shouldn't taste salty; if they are, continue to rinse and squeeze the pieces. Drain the cucumber in the colander, then lay them on a clean dish towel. Roll up the towel over the cucumber, and squeeze out as much liquid as possible.
Put the cucumber pieces in a bowl, add the fish sauce, lime juice and sugar and mix thoroughly. Cut the chilli into thin rounds, squeezing out and discarding the seeds as you go, then add the pieces to the bowl. Mix well then taste for seasoning and adjust, if necessary. Set aside while preparing the mackerel.
Lay the mackerel on a cutting board. Lift the gill then remove the head by cutting just behind it. Remove the dorsal fin by making a shallow slit into the fish along either side of the fin; pull it (and the attached bones) from the fish.
Place the fish with the stomach cavity facing you and look inside so you can see the ribs and backbone. Use a very sharp knife to slice the flesh along one side of the ribs, angling your knife slightly so it cuts towards the bones, rather than towards the skin, and don't cut through the backbone. When you've cut the ribs on one side of the entire length of the fish (stopping at the tail), turn the fish over and cut along the ribs on the other side. The ribs will be connected at the backbone.
Open the fish so the flesh lies flat on the cutting board and the ribs are sticking up. Carefully slice between the flesh and the backbone. Use kitchen shears to snip off the backbone where it's attached to the tail. Remove the backbone (and ribs) from the flesh.
Check to see that you've removed the fine bones that run along the stomach and backbone of the fish. Gently run your fingers over the exposed flesh to find any remaining bones and pull them out. Rinse the fish then pat it dry with paper towels.
Sprinkly salt lightly but evenly over the flesh and skin. Gently press the salt into the fish then leave at room temperature for 10 minutes.
Heat a lightly oiled, well-seasoned grill pan (preferably cast iron) over a high flame. Brush the fish skin with oil then lay it skin-side down in the very hot pan. Carefully press on the fish so it comes in contact with the pan. Sear the fish and cook for about two minutes.
Brush oil on the fleshy side of the fish then turn it over in the pan to cook that side. Because the fish is thin, it will take about four minutes in total to cook.
Put three fish or four fish on each plate and add some of the cucumber spaghetti. Garnish with fresh coriander and lime wedges, then serve.