In a pot, boil water with salt and cook spaghetti as instructed in the package. Drain. Make sure to keep about ½ cup of pasta cooking water.
Chop boiled octopus finely with a sharp knife.
Cut the eryngii mushroom in half if it’s long, and slice into smaller pieces.
Remove seeds from red chilli pepper and chop finely.
Heat the garlic, chili peppers, and olive oil in a pan over medium heat. Once the garlic is fragrant, add the octopus and cook for about 1 minute.
Reduce heat to low and add sake to the pan. Then, add the eryngii mushrooms. Continue cooking until mushrooms are tender. Turn off the heat.
Add cooked spaghetti and about ½ cup of pasta cooking water to the pan. Add the seasonings and mix well.
Place in plate and finally add the thinly cut shiso leaves on top.