Simple, subdued but umami-rich chilled soba is a quintessential Japanese noodle dish, and a favorite on hot, humid summer days. The classic version, called zaru soba, is a minimalist pairing of just noodles and a dipping sauce. Here, however, we include a few colorful garnishes to round out the meal, and we pour a soy-seasoned broth around the noodles instead of offering sauce on the side. Dashi, the base of this broth and a key ingredient in countless Japanese dishes, is a stock most commonly made with kombu (a type of seaweed) and katsuobushi (shavings of smoked, dried tuna). You can make your own dashi, but store-bought instant dashi, prepared according to the package instructions, works perfectly well. Look for it sold in boxes, bags or jars in the international aisle of the supermarket. Grated daikon, a variety of white radish, is an optional garnish for the soba, but we highly recommend including it. It adds an earthy piquancy that perks up the flavors. If you are using frozen shelled edamame that are already fully cooked, they need only to be thawed (no need to simmer then shock in ice water).