All you need is boiled buckwheat from yesterday's lunch (or today, for that matter), chicken or vegetable stock, squash or zucchini, mushrooms, chili pepper, some greens, and potato starch.
Heat up the stock and add chopped zucchini (or squash), mushrooms, and chili. When it starts to bubble, turn the heat down to a simmer. Chop the greens.
Take a tablespoon of potato starch and dissolve it in a small amount of cold water. When the vegetables are tender, add buckwheat and the starch to the soup.
Garnish with chopped greens. All done!