For the brine: Put the sea salt, demerara sugar, peppercorns, cloves, bay leaf, thyme and 3 1/2 cups of water in a pan and bring to the boil. Make sure the sugar and salt have dissolved, then remove from the heat and leave to cool.
For the pork belly and lentils: Place the pork belly in a plastic container with a lid. Pour the brine over the pork to cover and seal the container with the lid. Transfer to the fridge for 24 hours.
When ready to cook, preheat the oven to 300 degrees F.
Remove the pork belly from the brine and pat dry. Place the pork belly onto a wire rack suspended over a baking sheet. Roast until crisp and golden-brown, 2 1/2 to 3 hours. Allow the pork to rest for at least 30 minutes.
Heat a little oil in a saucepan set over a medium heat and add the bacon. Cook until crisp, then remove the bacon and add the onions to the pan. Fry until soft and translucent. Return the bacon to the pan. Stir in the lentils and herbes de Provence and cook for 1 to 2 minutes. Add the white wine and bring to the boil. Pour in the chicken stock and bring to a simmer. Reduce the heat and cook until the lentils are just soft, about 30 minutes.
For the salsa: Bring a pan of water to a boil. Blanch the mint and parsley in the boiling water for a minute. Remove the herbs from the pan, keeping the water boiling, and plunge them into ice-cold water. Add the cavolo nero to the boiling water and, after 2 minutes, transfer to the cold water with the herbs. When the leaves are cold, drain them thoroughly and squeeze to remove any excess water.
Chop together the cavolo nero, mint leaves and parsley leaves. Add the capers, anchovies and garlic. Put this mixture into a bowl with the salt, cayenne pepper, lemon zest and shallots. Mix in the olive oil to make a rough salsa.
To serve, slice the pork, spoon the lentils into a large serving bowl and serve with the salsa.