Thinly slice the onion. Julienne the shiso leaves, reserving some for garnish.

Separate the canned saba into fish and sauce. Roughly flake the fish.

Boil water in a pot, add salt, and cook spaghetti slightly shorter than the package instructions. Reserve 50cc of the boiling water.

Heat olive oil in a pan over medium heat. Sauté the onion and saba until softened. Add the reserved boiling water, canned sauce, mentuyu, salt, pepper, and spaghetti. Stir-fry until well combined.