Rehydrate the dried wakame in water, drain, and cut into bite-sized pieces. Remove the pits from the umeboshi and mash them with a knife.

Thinly slice the onion and soak it in water to reduce sharpness, then drain.

Boil water in a pot, add 1 tsp of salt (not listed in ingredients), and cook the capellini for 1 minute longer than the package instructions. Drain and rinse under cold water, then drain again.

In a bowl, mix the sauce ingredients (soy sauce, olive oil, and black pepper).