Trim the root of the shimeji mushrooms and separate them by hand. Slice the onion thinly and julienne the shiso leaves.

Make a slit in the tarako skin and extract the contents.

Place the steamed Chinese noodles on a microwave-safe plate, cover loosely with plastic wrap, and heat in a 600W microwave for 1 minute. Remove the wrap and loosen the noodles lightly.

Heat butter in a frying pan over medium heat. Once half melted, add shimeji mushrooms and onion, and sauté until softened.

Add the steamed noodles and tarako to the pan, stirring to combine. Season with mentuyu sauce, salt, and pepper, and mix well.