Place the rice in a sieve and rinse under a cold tap until the water runs clear. Tip the rice into a pan, add 600ml of freshly boiled water and bring to the boil.
Place the lid on, then turn the heat down to a whisper and cook for 15 minutes. Take off the heat and leave to steam with the lid on.
To cook the nasi goreng base, heat the rapeseed oil in a large frying pan on a medium flame and fry the onion, stirring, for 5 minutes.
Add the garlic and two-thirds of the chopped chillies, cook for 2 minutes more, then add all but 2 large handfuls (or about 150g) of the sprouts. Fry for 8 minutes, leaving them undisturbed for a couple of minutes at a time, so they get some colour on them.
Then stir in the tomato purée, kecap manis, salt and a tablespoon each of soy sauce and vinegar. Cook for another 5 minutes, then take off the heat.
To make the marinated sprouts, put the remaining raw sliced sprouts into a bowl with 1 tablespoon of soy sauce, 1 tablespoon of vinegar, the sesame oil, sugar and the remaining chopped chillies. Mix very well and set aside.
To finish the nasi goreng, put the sprout and onion pan on a medium heat and gently scoop in the steamed rice, folding it in until well mixed.
Heat through, stirring gently, for 5 minutes, until the rice is nice and hot, and season with salt to taste. Transfer to a big platter, scatter the marinated sprouts over the top, and serve.