When you cook Japanese noodle dishes, everyone should be ready at the dinner table, because cooked noodles start to soften and lose their taste and texture quite quickly.
To make the soup, put the mirin in a large pan. Bring to the boil, then add the rest of the soup ingredients apart from the dashi stock. Bring back to the boil, then reduce the heat to low. Skim off the scum and cook for 2 minutes. Strain the soup and put back into a clean pan with the dashi stock.
Remove the vein from the prawns, then make 5 shallow cuts into