Like seaweed-steeped dashi, the base for this soup comes together in minutes. Then you use the gingery, tea-flavored liquid to cook soba noodles, adding both body and a hint of buckwheat to the broth. Tempeh is a fermented soybean cake with a multi-faceted, tangy taste. The best way (we think) to enjoy it is crumbled and crisped a little, so it lends oo…