Because miso soup may be made with a variety of different ingredients, the best explanation in English may be just to call it 'Japanese stock soup'. The average person in the UK would not have a clue what miso soup is, so some explanation is necessary.
Miso soup is a japenese soup made with soya bean paste
Most english people would not know exactly what miso soup is as
1) its not as common in the uk
2) it can be in different varieties depending on the family/way its made
its basically a japenese soup made with stock and soya bean paste but you would need to tell the person hows its made to understand what it was exactly
You can call the soup "miso soup" and when you explain what it is, then you can explain that it is Japanese stock soup.
For example, you can say: I really like miso soup.
"miso soup"と言うことができます。それが何かを説明する時には、Japanese stock soupと言えます。
例：
I really like miso soup.
味噌汁が本当に好きです。
Miso is well known in many countries now as Japanese cuisine is very popular. Many will already be familiar with miso but if they need a description you can say the ingredients.
日本 
アメリカ合衆国 
イギリス 
南アフリカ 
オーストラリア 
