Combine all the ingredients for the marinade apart from the pork and groundnut oil, in a saucepan and whisk until smooth. Bring to a simmer, whisking to make sure the sugar has dissolved, then remove from the heat and allow to cool for 10 minutes. Add the pork after 10 minutes, cover and allow to marinate for 30 minutes. When ready to cook the pork, remove it from the marinade and pat dry; make sure you reserve the marinade. Heat the groundnut oil in a wok over a high heat until hot. Stir-fry the pork for 4-5 minutes, then add half of the reserved marinade and continue cooking over a reduce heat until it has reduced by half. Remove from the heat once the marinade has reduced by half. Arrange the spinach leaves in serving bowls. Spoon into the serving bowls. Serve immediately and eat with chopsticks.
