Mix the cooked barley with the koji and leave at room temperature for 2 days, covered with cling film, making to sure to mix it each day with clean hands.
Once the barley is inoculated, sprinkle with the salt and place into a glass preserving jar. Press down and leave it to ferment in a dark, room temperature space.
The above takes at least 1 year to ferment properly.
(If you are looking for a quicker fix, the other option is to ferment 200g barley by mixing with 2.5g of the salt and 100ml of water in a glass jar, pressing the mixture down firmly in a jar, then sprinkling with a pinch more of the salt and covering with a lid. This will be ready in 2 weeks at room temperature.)
This can be made with any mushrooms, either wild or cultivated.
Grab a jar with a lid, put the yogurt inside. Fill a medium sized cooking pot with water and put the jar inside. Turn the oven on 140c. When warm, put the pot with the jar in the oven, cover with a lid and slow-cook for 13 hours.
Strain the yogurt, then blitz with the olive oil and season with salt until very smooth.
Take some of the fermented barley and mix with the yogurt, it’s up to you the proportions - I like 1 part yogurt to 2 parts barley.
Sauté the mushrooms in butter, then serve with the fermented barley and yoghurt mix.
Shave some raw white mushrooms and garnish the dish alongside some sorrel leaves.