Combine soy sauce, lemon juice, mirin, 2 tablespoons water and bonito in small bowl. Let stand 30 minutes. Strain sauce through fine strainer; discard bonito shavings. (Can be made 4 hours ahead. Cover and let stand at room temperature.)
This is the type of soup that, at first glance, might seem a little…unexciting. But you’re underestimating the power of mushrooms, which do the heavy lifting.