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 lasagna soup delivers rich, baked-pasta flavor without an oven. Made with Italian sausage and spinach, it’s a fast, weeknight-friendly take on the classic.