Place the peas in a bowl, cover with water, then leave to soak in a fridge overnight
Rinse the peas, transfer to a saucepan and then boil gently, until soft (approx 45 minutes)
Drain, then leave to peas to cool. Once cool, add the koji and salt.
Blend until well combined. If using for a soup a smooth paste is best, but feel free to leave chunky if you're using the miso in other ways.
Press the miso into a clean jar, ensuring all air pockets are removed. Then lightly sprinkle surface with salt. Cover (eg. with a muslin secured with an elastic band), then place in a warm place (eg. in an airing cupboard) for 10 days.
After 10 days, scrape off the salt and transfer to a fridge. To make the soup, just stir a tablespoon of miso into a mug of stock.