If cooking the rice from scratch, cook according to pack instructions and set aside to cool.
Soften the onion in the butter, in a saucepan set over a low heat, for about 8 minutes. Set aside to cool.
Preheat the oven to 170°C, 150°C fan, 325°F, gas 3.
Sort through the cabbage leaves, setting aside about 12 large leaves. Roughly chop the remainder and spread across the base of a large, lidded casserole or saucepan. Cut any hard stem from the large leaves, keeping them as whole as possible.
Bring a large saucepanful of salted water to the boil and blanch the whole cabbage leaves for about 2 minutes, until soft and bright green. Plunge into cold water and drain thoroughly.
Combine the cooked rice, pork, beef, egg, garlic, oregano, cooked onion and plenty of seasoning
Holding a blanched leaf out flat, heap a generous 2 tablespoons of the meat mixture in the centre and fold in the edges, rolling the package up to make a tight parcel. Sit on top of the chopped cabbage leaves, seam-side down. Repeat until all the parcels are layered in snugly, seasoning with salt and pepper as you go.
Combine the beef stock and passata, pour over the rolls, cover with a lid and cook for 50 minutes – 1 hour. Cook the meat thoroughly until no pink is showing.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.