For the marinade, mix together the soy sauce, mirin, hoisin sauce, sesame oil and chilli flakes in a shallow dish until well combined.
Coat the pork in the marinade and chill in the fridge for 15 minutes.
Heat a wok over high heat and add one tablespoon of the oil. Once smoking add half the pork, shaking off the excess marinade, and stir-fry for 1-2 minutes.
Remove the pork from the wok with a slotted spoon and stir-fry the remaining pork for 1-2 minutes. Remove the pork from the wok.
Heat the remaining tablespoon of oil in the wok and add the garlic, chillies, one tablespoon of water and the onions. Stir-fry for 1-2 minutes. Cover with a lid and cook for a further 1-2 minutes.
Add the soy sauce and sugar and return the pork to the wok. Cook for a final minute. Garnish with the roasted peanuts and serve with steamed rice.