Best made with flakes of dried bonito (a dark, full‐flavored fish) and kelp (a strong‐flavored sea vegetable), page 602), both of which are readily available at every Asian market, this is still quite delicious without them.
Place the water and kelp in a small saucepan and bring to a boil; turn the heat to the lowest possible setting (the broth should stay hot but not boil from this point on). Put the bonito flakes in a tea ball or bag made of cheesecloth and place in the pan. Let sit for 5 minutes, then remove the kelp and bonito. If you are using chicken stock, omit the kelp, bonito flakes, and water, and begin by heating the chicken stock to the boiling point and then turning the heat to very low.
Put the miso in a small bowl; add 2 or 3 tablespoons of the broth and blend with a fork or small wire whisk until it is smooth.
Stir the miso mixture into the soup.
Add the tofu or chicken and scallions. Let sit for 1 minute before serving.
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