Thinly slice the onion and celery and finely chop the garlic. Chop the carrots and bell pepper into small chunks.
Heat the olive oil in a large frying pan or casserole pan over a medium-low heat. Add the onion, celery, carrots and pepper and gently sauté for about 10 minutes, stirring occasionally, until the vegetables are tender.
Add the garlic, oregano, cumin, smoked paprika and tomato purée. Stir for about 1 minute, until fragrant, then add the chopped tomatoes and stock cube. Pour in enough water to cover the vegetables well.
Bring to the boil, then reduce the heat and simmer for about 15 minutes, until the vegetables are tender and you have a thick, rich sauce.
Meanwhile, cut the duck liver into bite-sized chunks. Add the liver to the sauce together with the drained lentils and chopped green beans.
Cover with a lid and cook for about 5 minutes, or just until the liver is cooked through. Take care not to overcook it, as duck liver can become tough and rubbery.
Stir through the chopped parsley and serve with pasta, rice, gnocchi or buckwheat.