Contrary to what most believe, the soup that is served is NOT miso soup. Miso soup is essentially a Japanese soup in which dashi (stock made with seaweed and sardines) is combined with miso (fermented soybean paste). This Japanese onion soup is more basic, much easier to replicate, and in my opinion, SO much tastier!
It's fairly simple to make because there isn't much prep work and once it's boiling, you just let it simmer. How long it simmers is up to you, but no less than 45 minutes. The vegetables do not need to be diced or chopped precisely, as they are the flavoring and will be strained out. Also, taste occasionally as you may need to add more water. I added an additional 1½ cups of water because when I tasted mine, it was a little salty. In total, I used 9½ cups of water.
Japanese Onion Soup
1 small onion, quartered
2-3 carrots, roughly chopped
1 stalk celery, halved
2-3 cloves garlic, halved or quartered
½ inch of ginger, roughly chopped
2 tbsp. beef bouillon
2 tbsp. chicken bouillon
3 mushrooms, quartered
8 cups water
Optional toppings include scallions, french fried onions, thinly sliced mushrooms, etc.
Place all ingredients into a large saucepan or stock pot. Bring to a boil over high heat. Once boiling, cover and reduce heat to medium. Gently simmer for at least 45 minutes. (You may simmer anywhere from 45 minutes to 2 hours or more!) I recommend simmering for about an hour and a half. Remove from heat and strain the vegetables from the liquid. The vegetables should be discarded because they should have already given up their flavor into the stock. My favorite topping is french fried onions and scallions while my boyfriend prefers the mushrooms.