Heat the butter and oil in a heavy pan over a medium heat, and sweat the onion for a few minutes until soft and starting to caramelize. Add the meat and sear for 10 minutes until browned on all sides. Add the peas and let them cook for another few minutes. Pour in the stock and tomato passata and season with salt and black pepper. Simmer very gently over a medium heat, uncovered, for about 1 hour until the sauce has reduced and the meat is tender.