The blowfish has struck again. Seven daredevil diners in Tsuruoka city, northern Japan, are recovering from severe poisoning after eating grilled blowfish testicles on Monday night. The men experienced breathing difficulties and limb paralysis, and three of them remain in hospital.

Blowfish, or pufferfish, are fiendishly difficult to prepare safely because many of their internal organs contain a toxin that is said to be 10,000 times more poisonous than cyanide.

"Japanese chefs must have a licence to prepare blowfish in Japan," says a member of the sushi team at esteemed Japanese eatery Nobu Berkeley St, in London. "This is very difficult to get and takes years of training." The chef involved in this week's disaster was not licensed and is now in the hands of the police.

Do they serve this dangerous delicacy at Nobu? "It is illegal to import blowfish to the UK and eat it here," says our chef gravely.

Takanori Kurokawa, executive head chef at Soseki, London, held a blowfish licence about 12 years ago when working in Tokyo. "The testicles aren't normally eaten in Tokyo or Osaka," he says, "as it's against local laws, but there are places where people eat them, because they're tasty."

According to Kurokawa, there's a lot of toxin in the testicles, but pickling them for up to a year will dilute the poison. Nevertheless, he says, "I don't agree with eating this part and wouldn't recommend anyone else doing so."

The blowfish sashimi, or "fugu", on the other hand, "is delicious and usually eaten in winter. It is gelatinous, doesn't smell fishy, and has the most umami [that savoury flavour often associated with monosodium glutamate] of all the fish."

Nearly all fatalities occur when untrained persons prepare and eat them. But, purely in the spirit of curiosity (ie don't try this at home), how does one go about preparing a blowfish?