Heat the oil in a large skillet. Sauté the onion over medium heat until golden. Add the garlic and broccoli; sauté, covered, until the broccoli is bright green, 2 to 3 minutes.
Add the tomatoes, beans, and ginger. Simmer over low heat for 10 minutes.
In a small bowl, combine the miso, arrowroot, and about ¾ cup warm water; whisk together until smooth. Pour into the skillet along with the sesame oil. Simmer gently until the liquid has thickened, just another minute or so.
Spread the cooked grain of your choice on a serving platter, then top with the bean and broccoli mixture, allowing a bit of the grain to show around the edges. Sprinkle the scallions over the top and serve at once.