Choose the mug/jar that the cake will be cooked in. A coffee mug is the ideal size. A smaller mug/jar will work, but it might spill over a bit in the microwave.
In the mug/jar, combine the rice flour, millet flour, sorghum flour, sugars, baking powder, ginger, vanilla, cinnamon, and salt. Stir until well mixed.
Stir in the diced apple.
Add the oil and water to the mug/jar. Stir until the flour is moistened.
Microwave on full power for 1 minute 15 seconds. Check the doneness of the cake. If it is under cooked, microwave for up to 30 more seconds.