Cook the rice as per the packet instructions. This usually takes around half an hour.
Put the sugar and water into a saucepan. Bring to a simmer and stir until the sugar has dissolved.
Add the soy sauce and ginger and bring to a boil. Reduce to a simmer and cook for 5 minutes or until starting to thicken.
In a small dish or ramekin, mix together the cornflour and 1 tablespoon water until completely smooth. Pour into the saucepan while stirring the teriyaki sauce to prevent lumps from forming. Add the rice vinegar and stir again. Cook for a further 3-4 minutes or until the sauce is thick and glossy. Remove from the heat.
Heat the oil in a frying pan and add the seitan. Fry for a few minutes until starting to crisp up on the outside, then add the teriyaki sauce. Stir to coat and cook for one or two minutes more until hot through.
Divide the rice between two bowls and add a handful of spinach on the side. Top the rice with the teriyaki seitan, then sprinkle with the sesame seeds and spring onions.