Discard any tough outer leaves and stalks from the Chinese leaves and pak choi and wash well. Drain thoroughly and pat dry with kitchen towel. Shred the Chinese leaves and pak choi lengthways. Set aside.
In a small bowl, blend the cornflour with 4 tbsp of water. Add the soy sauce, sugar, vinegar, orange juice and tomato puree and stir until blended thoroughly.
Pour the sauce into a small saucepan and bring to the boil. Simmer gently for 2-3 minutes, or until the sauce is thickened and smooth.
Meanwhile, heat a owk or large frying pan and add the sunflower oil and butter. When melted, add the prepared Chinese leaves and pak choi, sprinkle with the salt and stir-fry for 2 minutes. Reduce the heat and cook gently for a further 1-2 minutes or until tender.
Transfer the Chinese leaves and pak choi to a warmed serving platter and drizzle over the warm sauce. Sprinkle with the toasted sesame seeds and serve immediately.