Cold soba noodles are often eaten in summer and served on a bamboo tray with a dipping sauce. The Japanese love the taste of the noodles; the sauce enhances their flavour.
To make the dipping sauce, mix all the ingredients in a small pan. Bring to the boil, and cook for 2 minutes. Reduce the heat to medium, and cook for a further 2 minutes. Strain through muslin (cheesecloth). Cool, then chill.
Soak the spring onions in ice-cold water in a bowl for 5 minutes. Drain and squeeze out the excess water.