Remove the pit from the umeboshi and mince it with a knife. Slice the shiso leaves thinly.

In a large bowl, mix the minced umeboshi with mentsuyu and olive oil.

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

Add the cooked spaghetti and dried shirasu to the bowl with the sauce and toss well.