Start by cooking the buckwheat. Place it in a saucepan with twice its volume of water. Crumble in half of the stock cube. Bring to the boil, skim off any foam, then reduce the heat and simmer until the liquid has been absorbed. Cover with a lid and allow to rest while you prepare the ragu.
Meanwhile, heat the olive oil in a large frying pan or casserole dish over medium heat. Add the onion, garlic, celery, carrot and red pepper. Cook gently for 8–10 minutes, stirring occasionally, until softened.
Add the Italian herbs and chopped tomatoes. Crumble in the remaining stock cube and add enough water to just cover the vegetables. Cover and simmer gently for about 15 minutes to allow the flavours to develop.
Stir in the lentils and chopped kale. Add a little more water if needed — the consistency should be thicker than a stew but still moist. Cook for another 5 minutes, until the kale has wilted.
Cut the duck liver into bite-sized pieces and add them to the pan. Stir gently, cover and cook for about 3–5 minutes, until just cooked through. Avoid overcooking, as the liver can become tough.
To serve, place the cooked buckwheat in a large serving bowl and spoon the ragu over it. Season to taste and mix gently if desired.
Finish with freshly chopped parsley and serve immediately.