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 sauce is slightly magical. The texture cloaks pasta much like a traditional meat sauce does, and the flavors are deep and rich, but it’s actually vegan!