Preheat the oven to 180 C/350 F/Gas Mark 4, 10 minutes before cooking. To make the sauce, heat the oil in a heavy-based saucepan, add the spring onions and cook for 2 minutes or until softened.
Add the tomatoes, soy sauce and mint to the saucepan, bring to the boil, cover and then simmer for 10 minutes. Season to taste with pepper. Reheat when required.
Meanwhile, blanch the cabbage leaves in a large saucepan of lightly salted water for 3 minutes. Drain and refresh under cold water. Pat dry and reserve.
Heat the oil in a small saucepan, add the celery, carrot and pork mince and cook for 3 minutes. Add the mushrooms and cook for 3 minutes. Stir in the Chinese five spice powder, rice, lemon juice and soy sauce and heat through.
Place some of the filling in the centre of each cabbage leaf and fold to enclose the filling. Place in a shallow ovenproof dish seam-side down. Pour over the stock and cook in the preheated oven for 30 minutes. Serve immediately with the reheated tomato sauce.