Wash the nameko mushrooms briefly and drain the water. Remove the pit from the umeboshi and mash it into a paste using a knife. Cut off the stems of the shiso leaves and slice them thinly.

Place the nameko mushrooms and 3 tablespoons of water in a microwave-safe container. Cover loosely with plastic wrap and microwave at 600W for 1 minute and 30 seconds. Rinse under cold water and drain.

In a bowl, mix the natto with its included sauce. Add the nameko mushrooms and umeboshi paste, then toss to combine.