Finely chop the onion, celery, carrot, red pepper, and garlic.
Heat the oil in a large casserole dish or deep pan over medium heat. Gently sauté the vegetables until softened.
Add the tomato purée and dried herbs and cook for about 1 minute, stirring.
Pour in the tinned tomatoes along with one tin of water. Add the stock cube and bring to a gentle simmer.
Cover and cook for 15 minutes.
Add the drained lentils and chopped kale or cavolo nero. Bring back to the boil, then cover and cook for another 5 minutes.
Cut the duck liver into bite-sized pieces and add them to the pan.
Cover and cook for 3–5 minutes, until the liver is just cooked through. Avoid overcooking, as it will become tough.
Remove from the heat and serve warm, scattered with fresh parsley.