Nutty, wholesome Japanese buckwheat noodles and bok choy are sauced with a puree of toasted walnuts and miso that delivers a double hit of umami. The starchy water that results from cooking soba is called soba yu; we use a little of it to help the sauce blend smoothly and also cling to the noodles. You can use either sweeter, milder white miso, or saltier, earthier red miso—or even a blend if you happen to have both types.