Season the lamb fillets with salt and black pepper.
Heat a frying pan until hot, add one tablespoon of the oil and fry the lamb on all sides until browned. Remove from the pan and set aside.
Heat a wok until hot, add the remaining oil, then add the chillies, garlic, ginger and spring onions and stir fry for one minute.
Add the leek and spring cabbage and stir fry for another two minutes then add the fish sauce, tamarind paste and coconut milk and bring to a simmer.
Cut the lamb into thick slices then add to the vegetables and toss through to combine.
Cook for one more minute then remove from the heat and add the mint, coriander and mizuna leaves and toss to combine. Add the lime juice and check the seasoning.
To serve, lay a banana leaf into each of four soup plates then spoon the stir-fry on top.