Rinse the oyster meat in cold water.
Place in a medium saucepan with the water, fish stock, sea salt, garlic, spring onion and ginger and bring to the boil over medium-high heat. Reduce the heat, partially cover and simmer for 30 minutes.
Mix the cornflour or kudzu with a little cold water to form a smooth paste. Add the shoyu and maple syrup, then gradually pour the mixture into the pan, stirring continuously. Simmer for another 10 minutes to allow the sauce to thicken, stirring occasionally to prevent sticking.
Pour the thickened sauce through a sieve into a sterilised jar with an airtight lid (discard the solid residue). Refrigerate once cool and keep in the fridge for up to 10 days.