To make the base; cut the celery and carrots into medium sized chunks. Cut the fennel in half, remove the core and cut each half into six wedges. Add a good splash of extra virgin olive oil to a medium-sized pot and begin to sweat the vegetables over a medium heat with a pinch of salt. Wash the lemon, peel the rind and keep aside, remove the pith and dice the flesh and reserve.
Rinse the lentils in a sieve.
Add the garlic to the vegetables in the pot, add the bay leaves and cinnamon stick. Squeeze pieces of sausage meat from their skins straight into the pot. Add the lentils, lemon rind and passata and then cover with a centimetre of chicken stock. Bring it all to the boil and then turn down to a simmer. Continue to cook over a low heat adding a little more chicken stock as needed. It will take about 30 minutes to cook the lentils. Keep adding stock until they are just soft. Turn the heat off, stir through the chopped cavolo nero leaves and lemon flesh, and then check the seasoning.
Plate and add a liberal glug of Mount Zero olive oil just before serving.