In a small, heavy saucepan, heat the buckwheat groats over medium-high heat. Swirl the groats in the pan, toasting them until they are crackling, hot to the touch, and fragrant, about 5 minutes. In a wire-mesh strainer, wash the hot buckwheat quickly and drain thoroughly. Put the 1½ cups water in the pan and bring to a boil. Add the buckwheat, return to a boil, cover tightly, and reduce the heat to the lowest setting. Cook for about 20 minutes, until all the liquid is absorbed. Take the pan off the heat and let stand for 5 minutes, then transfer the cooked grain to a bowl, cover, and let cool to room temperature.
In a large measuring cup, whisk the miso, canola oil, and vinegar until smooth. Whisk in the sesame oil, ginger, garlic, pepper flakes, and honey. Pour the dressing over the cooled buckwheat and toss to coat.
To serve, spread the buckwheat on a platter, and top with the broccoli, carrot, and scallions. Sprinkle the cashews over the salad and serve. Alternatively, mix the veggies into the grain and chill.