Remove bones from mackerel and cut into bite-sized pieces. Thinly slice the green onion.

Trim the root of the shimeji mushrooms and separate them. Mince the garlic and halve the chili pepper, removing seeds.

Boil water in a pot, add salt (not listed in ingredients) and spaghetti, cook according to package instructions, then drain.

Heat olive oil, garlic, and chili pepper in a frying pan over low heat until fragrant. Add mackerel and stir-fry over medium heat.

Once the mackerel changes color, add green onion and shimeji mushrooms, and stir-fry briefly.

Add canned tomatoes, salt, and pepper. Simmer for about 5 minutes until the sauce thickens.