How to Make It
Preheat oven to 350°F. Spread walnuts on a rimmed baking sheet and toast until a shade darker and nutty-smelling, 10 to 12 minutes. When cool enough to handle, roughly chop and toss with 1 tablespoon of the oil; season to taste with salt and pepper.
Meanwhile, bring a large pot of salted water to a boil. Add green beans and cook until bright green and crisp-tender, about 3 minutes. Drain beans and transfer to a bowl filled with ice water to cool; drain and pat dry.
Whisk miso, mustard, vinegar, honey, 1 tablespoon water, ½ teaspoon salt, remaining 1 tablespoon oil, and several grinds of pepper in a large bowl. Add green beans and toss to coat. Transfer to a platter and top with toasted walnuts.