Soak brown rice overnight, then drain and transfer to a heavy pot with 8 cups water. Cook for 40 minutes until tender, stirring occasionally.
Meanwhile, make Kombu Tsukudani by simmering leftover dashi kelp and shiitakes in soy sauce, mirin and sugar until soft but bouncy texture.
Once porridge base cooked, mix in sliced daikon, carrots and white onions. Cook 5 more minutes.
Finish by mixing in chopped green onions and garnishing with mitsuba. Allow to sit before serving.
Serve porridge in bowls with tsukudani mix-ins and salty sides like umeboshi for contrast.