Bring the stock to a rolling boil, stir in the marmite or soy, pour over the chunks.
Cover and leave for 10 minutes.
Drain, cool slightly, then squeeze out as much liquid as possible for the best texture.
Fry the chunks in a generous splash of oil over medium high heat, letting them sit before stirring, until deeply golden and crisp.
Season well and push to one side.
Fry the spring onions, chilli, ginger and garlic in the empty side for 1 to 2 minutes until fragrant, then toast the pepper briefly.
Stir in the ketchup, soy, sriracha, vinegar and sugar, bubble for a minute, then toss everything together.
Pour in the slurry and cook for 1 to 2 minutes until glossy and sticky, loosening with a splash of water if you like it saucier.
Taste and adjust the seasoning, then serve hot on rice.
