Thinly slice the onion and garlic. Peel and chop the carrot and sweet potato into bite-sized pieces.
Heat a little olive oil in a large saucepan and sauté the onion for about 5 minutes, until softened.
Add the garlic, oregano, carrot, sweet potato, tomatoes and cabbage. Stir for a minute, then add the passata, stock cube and enough water to cover the vegetables.
Bring to the boil, then reduce the heat and simmer until the vegetables are nearly tender. Dice the courgette, add it to the stew and cook for another 5 minutes.
Meanwhile, trim the duck liver if needed and cut it into small, even-sized pieces. Add it to the stew and stir gently for a couple of minutes.
Add the drained cannellini beans and continue cooking gently until the liver is cooked through. Avoid overcooking it, as it can become tough and rubbery.
Turn off the heat, cover the pan and allow the stew to rest briefly. Stir in the cooked buckwheat and warm through gently if needed.
Serve hot, scattered with fresh chopped parsley.