Before starting, ensure all utensils and bowls you use are scrupulously clean. Hands too!
Cut or tear the oyster mushrooms lengthwise (down the lines of the gills) into thin slivers. Pile them into a steamer and steam for 20-30 minutes on a fairly high heat. They will be fine and stringy, but tough, when finished. Allow to cool enough to handle.
While the mushrooms are cooling pound the salt and garlic to a fine paste with a pestle and mortar.
Rinse the cooked rice to break up any clumps and squeeze or press it to remove excess water. Pop the rice into the mortar and pound lightly with the garlic paste. The goal isn't to make it into a paste, but to break the grains down only slightly.
Once the steamed mushroom shreds have cooled you must rid them of excess moisture by wringing them out in muslin or another thin cloth. Pressing in small portions will make this easier. Wring as much liquid out as possible (tip: save the liquid and use as a replacement for vegetable stock in recipes).
Combine the rice and garlic paste with the mushrooms in a bowl and knead with your hands to combine. Divide them into four portions and wrap them in squares of cling film to form small sausage shapes.
Remember, these would be fermented at temperatures around or above 30 degrees Celsius in Thailand, so find a warm spot to leave them to ferment. In the winter I often wrap them in a tea towel and leave them under a radiator. Another option is an airing cupboard. In ideal conditions these could be done in 24 hours, but I find anywhere from 3-7 days is right in the English climate. They should taste salty, slightly sour, and pungent from garlic when done.
You can eat them raw or fry them briefly, which is my preference because it imparts some oiliness that pairs well with the sides you should consume the sausages with: deep fried cashews and/or peanuts, raw cabbage, thinly sliced ginger, bird's eye chillies, chopped shallots, fresh lime, and sprigs of coriander.