Peel 1 carrot, 2 celery stalks, 1 onion or 2 spring onions, 1 leek and about 4 cm ginger in rough pieces.
Heat some flavor-neutral oil, sauté the vegetables until they get some color.
Pour in cold water to cover, add a piece of kombu seaweed (the dried piece of seaweed in the picture), and turn down to medium heat.
When the water simmers, fish out the seaweed and discard it.
Now add about 2 dl mirin, 1 dl sake and a bit of rice wine vinegar.
Let it reduce until there is about 3-4 dl liquid left, strain out the vegetables and add 4-5 tsp salt and 2-3 tsp sugar.
It needs to be very salty. The reduction took about 1 hour.