Start at the counter, not at the knife

Most disappointing homemade sashimi is not ruined by bad cutting. It is ruined in the shop, before the fish ever reaches your kitchen. A perfectly sharp knife cannot rescue a tired fillet, and no amount of practice will make dull, watery flesh taste of the sea. So before we talk about blades and angles, let us talk about choosing well. Spend twice as long at the fish counter as you do at the chopping board and your sashimi will improve overnight.

Reading freshness: what to look for

Fresh fish announces itself. If you are buying whole, work through this checklist in order — it takes seconds once you know it.

• Eyes: clear, dark, domed and glossy. Cloudy, sunken or milky eyes mean the fish is several days old.
• Gills: bright pink or cherry red, clean and damp. Brown or greyish gills with a slimy feel are a firm no.
• Smell: clean, mineral, faintly sea-like. Any trace of ammonia, bleach or sour milk is a sign of bacterial activity.
• Flesh: press a thumb into the shoulder and watch it spring back. A lasting dent means the protein has started to break down.
• Scales and skin: scales tight and shiny, skin glistening with a natural sheen rather than a dull, dry film.

For fillets, the same principles apply. Look for flesh with a translucent quality, well-defined flakes and no gaps where the muscle has separated. A dark, bruised bloodline running along the centre should be trimmed away, but a small amount is normal in oily fish such as mackerel.

Buying raw fish in the UK

Here is a truth worth knowing: "sushi grade" is not a regulated term in Britain. It is a marketing phrase, not a safety guarantee. What matters is provenance and temperature history. Ask your fishmonger plainly whether the fish was blast-frozen at sea or on landing, and when it was caught. Small coastal boats landing day-boat fish are often excellent, provided you are buying it the day it comes in.

On parasites, official guidance for eating fish raw is straightforward: freeze it first unless you trust the supply chain entirely. The Food Standards Agency advises freezing at -20°C for at least 24 hours, or at -35°C for at least 15 hours, which kills most parasites. Domestic freezers usually sit around -18°C, so give it 48 hours to be safe. Avoid freshwater fish raw altogether — the parasite risk is genuinely higher. Farmed Scottish salmon, farmed sea bass and bream, tuna, yellowtail and hand-dived scallops are all sensible starting points.

Temperature, hygiene and the home kitchen

Sashimi is a cold dish, and warmth is the enemy. Fish should sit between 0°C and 4°C right up to the moment you slice it. Keep it on the bottom shelf of the fridge, in a tray, loosely covered with a clean cloth or kitchen paper rather than sealed in cling film, which traps moisture and dulls the surface. Work in short bursts — no more than twenty minutes out of the fridge at a time.

• Chill your board, your knife and your serving plate in advance; a cold surface slows the enzymes that turn texture floury.
• Use a separate board from your everyday chopping, and wash hands and cloths between tasks.
• Dry the fish thoroughly with kitchen paper before slicing — surface water dilutes flavour and makes cuts ragged.
• Cook and eat within 24 hours of purchase; sashimi does not improve with keeping.

The cut is the dish

A long, thin, single-bevel knife is the traditional tool, but a very sharp sujihiki or even a well-honed carving knife will do. Sharpen it properly and strop it; you want a blade that glides, not one you have to push.

Slice with a single, drawing stroke from heel to tip, letting the length of the blade do the work. Sawing tears the muscle fibres and leaves a ragged edge that tastes limp. Cut across the grain, so the fibres are shortened, and angle the blade slightly rather than pressing straight down.

• Hira-zukuri: thick, flat slices, roughly 1cm — ideal for tuna, salmon and other rich fish.
• Usu-zukuri: paper-thin, almost translucent slices for bream, squid and lean white fish.
• Kaku-zukuri: neat cubes, useful for firm fish and for practising consistent thickness.
• Sogi-zukuri: an angled slice for skin-on fish such as mackerel, where the skin is part of the pleasure.

Trimming, finishing and serving

Before slicing, run your fingertips along the fillet and pull any pin bones with tweezers. Trim the dark bloodline and any silver membrane, which turns bitter and chewy. For mackerel, wipe the blade with a little rice vinegar between cuts to keep the skin clean and the flavour bright.

Arrange slices overlapping, slightly fanned, on a chilled plate. Take the plate out of the fridge ten minutes before serving — sashimi should be cool, never ice-cold, or the aroma is muted. Serve with freshly grated wasabi, good soy and pickled ginger, and slice just before you sit down. That single habit, more than any technique, is what separates a competent plate from a beautiful one.