Miso is, as many people know, a Japanese paste made from fermented soy beans, occasionally mixed with rice or barley. It comes in varying colours, the darker generally means more salty and with a deeper flavour. I personally like the darker kinds of miso has they have a richer flavour but you can use all kinds of miso for soups. Traditionally, the stock, called dashi is made from flakes of dried and smoked bonito (a kind of tuna), or from a type of seaweed called kombu. However, you can also buy sachets of powdered dashi, which is what I used here.
You can put all sorts of things into the basic stock/miso mixture, but I think that soaked dried edible seaweed is pretty essential and cubes of tofu and mushrooms also work well.
The soup is pretty simple to make if you use dashi powder. You dilute the powder with boiled water and dissolve a tablespoonful or two of miso in the stock base. Then you add in your seaweed, raw fresh mushrooms (I used shimeji mushrooms), cubes of medium tofu and whatever else you fancy. I limited myself here to adding beansprouts and some chopped Chinese chives. You don’t want to overload the soup with too many things. You could add some grated daikon radish or maybe some grated raw carrots.