Mix the oats with the soy kefir and leave to soften for 10 minutes.
Line the heatproof container with baking paper while the oats soak.
Wrap the steamer lid in a clean kitchen cloth to catch water drops, with the ends secured on top and away from the flame.
Bring the water in the steamer to a boil so it is steaming before you add the batter.
Stir the soaked oats thoroughly for about 2 minutes, breaking up the flakes to make a smoother mixture.
Whisk the egg, sugar, salt and baking powder into the oat mixture for 2 minutes.
Pour the batter into the lined container.
Steam the batter in the covered steamer over high heat for about 12 minutes.
Insert a skewer into the centre to check for doneness; if it comes out with wet batter, continue steaming until it comes out clean.
Remove the cake from the heat and let it cool slightly before cutting.
Storage and safety
Wrap and refrigerate leftovers promptly, within 2 hours of cooking, or within 1 hour if the room is above 32°C (90°F). Eat the next day, or freeze for up to 1 month. Steam again until hot throughout before serving.