Here's a recipe which I devised while trying to diet the otaku way in grad school. It's not authentically Japanese, but there are several similar authentic soups. Miso soup is just dashi and miso; this is miso soup with oomph.
Put everything* in a pot, add water to almost cover, and simmer until it's all hot. All of the ingredients are precooked, so don't worry about undercooking and poisoning yourself. This recipe makes one big serving of about 450 calories.
* Some miso recipes suggest adding the miso after the soup has stopped boiling. Personally, I can't tell the difference, but I'm not the most discriminating of gaijin.
You can add just about anything else you like. Chopped scallions and Korean rice cakes (flat white slices of rice paste about an inch across, sold frozen in bags) go well with this recipe.