Heat the oil in a large saucepan and add the chopped bacon. Cook until well browned (8-12 min).
Add the onion, carrot, celery, leek and garlic to the pan, season with black pepper and give it a stir. Saute, stirring frequently, until the vegetables are starting to soften and brown (10-12 min).
Stir in the smoked paprika and cook for a further minute.
Add the soaked lentils and stir to combine with the rest of the ingredients.
Pour in 1 litre of the stock, stir, then bring to a boil. Boil for 10 minutes then reduce the heat and bring down to a simmer.
Put a lid on the soup and simmer until the lentils are soft (approx 25 minutes). Stir occasionally to ensure nothing is sticking to the bottom of the pan.
Stir in the cavolo nero or kale, replace the lid and cook for a further 5 minutes.
Take the saucepan off the heat and, using a stick blender, blend to your desired consistency (you can also leave it chunky if you prefer). Add more of the stock to make the soup thinner if needed.
Taste the soup and add salt and more pepper if liked.
Serve the soup, adding a swirl of cream, soured cream or yogurt and chopped parsley if liked.
Can be cooled then refrigerated or frozen, without the garnishes.