Clean the prawns. Twist the head off carefully, leaving the rest of the shell on. The intestinal vein that runs along the back of the prawn should be visible and just sticking out. Give it a firm pull if you prefer to eat your prawns without. If you prefer your prawns completely without shell on, by all means, strip it naked, but keep the head and shell for the broth for both methods.
When done, place the prawns to one side. Then, give all the heads a clean by squeezing the gunk out and rinsing them.
Then place all the heads and shells if you've removed, them into a saucepan along with the dashi (or water) and bring to a gentle boil. Simmer for 15 minutes, removing any nasty foam that collects. Strain into a clean saucepan. NOTE: not the prawns themselves, just the shells and heads.
Let's make the Miso Soup
Place this saucepan with the strained dashi (stock) on medium heat. Add the prawns, cabbage, ginger and the white part of the spring onions to the dashi (or water), along with the soy sauce and rice vinegar. Bring to a simmer and cook for about 3 minutes until your prawns are just about done.
Add the udon noodles, cook for a minute and take off heat.
Add the spinach, give it a stir to gently heat through.
Finally, add the miso, check the seasoning, divide into 2 bowls and top with the green part of the spring onions and chilli slices if using.