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1
Microwave sweet potatoes until tender and easily pierced with a fork, about 5 minutes. Set aside to cool slightly. If desired, peel and discard jackets. Cut into medium cubes and allow to cool to room temperature.
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2
Meanwhile, on a microwave-safe plate, drizzle walnuts with just enough olive oil to lightly coat, toss well, and arrange in a single even layer. Microwave in 1-minute intervals at high power, tossing walnuts between intervals, until toasted and fragrant, about 3 minutes. Let cool.
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3
In a medium saucepan, combine water with kombu and cook over medium heat until just shy of a simmer. Remove and discard kombu using tongs. Remove from heat, add bonito flakes, and let steep for 10 minutes. Strain dashi into a heatproof container through a fine-mesh strainer. Discard bonito flakes. (See note.)
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4
In a small bowl, whisk 1/4 cup dashi together with soy sauce (this is now shoyu-dashi). Reserve remaining dashi for another use. (It's good just as a warming broth, or can be used to make a
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5
.)
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6
In a clean small bowl, whisk together 3 tablespoons shoyu-dashi with 2 tablespoons olive oil. Reserve remaining shoyu-dashi for another use. (It's delicious as a broth for poached eggs, steamed fish, or simple steamed vegetables.)
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7
In a salad bowl, combine sweet potato, walnuts, and arugula. Dress with shoyu-dashi/oil mixture, tossing well to coat. Season with salt and pepper and serve.
