Mackerel so Fresh It Might Swim Back

1. 01

   Prepare the pickle (do this the night before if you can)

   Slice the cucumber paper-thin on a mandoline — you want transparency, not chunks. Layer it with salt in a colander and leave it to weep for 15 minutes. Gently squeeze away excess moisture with your hands, not a towel (you'll bruise it). Warm the vinegar with 150ml water and a pinch of salt until it steams; pour it over the cucumber in a jar and let it cool. Refrigerate overnight, or at least 4 hours.

   ~15 min
2. 02

   Fetch the fish and check it twice

   Place your mackerel fillets on a board lined with kitchen paper. Look at them: they should gleam like chrome. Smell them: they should smell like the sea, not fishy — if they're high, they're off. Run your fingertips along the flesh to feel for any pin bones lurking under the surface. Remove them with tweezers if you find them (you will). Pat the fish dry.

   ~5 min
3. 03

   Cut the tartare

   Use a very sharp knife — dull knives bruise fish and release juices you want to keep inside. Slice the mackerel at a shallow angle into pieces no larger than a pencil eraser. Work quickly and chill the knife between cuts if the kitchen is warm. You want clean cuts, not mush. Pile the tartare on a cold plate and drizzle with the rapeseed oil. Taste a pinch: season with fleur de sel and one crack of pepper. Add more salt if needed — raw fish forgives nothing.

   ~8 min
4. 04

   Grate the horseradish

   Peel the horseradish root under cold running water — it's peppery and will make you cry, so ventilation helps. Grate it on the finest side of your box grater directly over each plate. Use a light hand: horseradish is a accent, not a assault. If it's too fierce, you grated too much.

   ~3 min
5. 05

   Assemble and plate

   Drain the pickled cucumber of excess vinegar — don't flood the plate. Arrange a few slices alongside the tartare. Scatter fresh dill fronds over the top. Finish with a final whisper of salt and cracked pepper. Serve on a cold plate — literally cold, chill it in the freezer while you're prepping.

   ~4 min

The first bite is all mackerel — clean, oily, salty from the sea — then the horseradish hits like a slap from a cold flannel, clearing your head. The pickle follows, vinegary and snappy, cutting through the richness of the oil. There's sweetness in the fish, bitterness from the dill. By the last forkful, it's all integrated: a slap, a hug, and a memory of why Cornwall matters.

• ·Freeze your knife and plate for 20 minutes before service — cold is your friend with raw fish
• ·Don't trust a fishmonger who won't tell you when the fish came in; if they hedge, walk
• ·Horseradish loses punch after grating; grate it five minutes before plating, not an hour