Season the lamb fillets with salt and black pepper.
Heat a frying pan until hot, add 15ml oil, and brown the lamb on all sides.
Remove lamb from the pan and set it aside.
Heat a wok, add the remaining 15ml oil, and stir-fry chillies, garlic, ginger, and spring onions for one minute.
Add leek and spring cabbage and stir-fry for two more minutes.
Stir in fish sauce, tamarind paste, and coconut milk and bring to a simmer.
Slice the lamb into thick pieces and toss into the vegetables.
Cook for one minute, remove from heat, and toss in mint, coriander, and mizuna.
Add lime juice and adjust seasoning before serving on banana leaves.