Peel and finely chop the onion. Peel and crush or grate the garlic.
Warm a large pan over a medium heat for 1 min. Add 1 tsp oil and the pork stir-fry strips. Fry for 3-5 mins, stirring once or twice, till the meat is browned all over. Lift out of the pan and set aside on a plate.
Add the onion to the pan with 2 tbsp cold water and a pinch of salt and pepper. Cook and stir the onion for 5 mins to just soften it. Stir in the garlic.
Add 2 tsp each of the smoked paprika, fennel seeds and ground cumin to the pan. Give it a good stir. Tip the pork strips and any juices from the plate into the pan.
Drain the butter beans and add them. Tip in the chopped tomatoes. Fill the can with warm water (approximately 400ml) and add to the pan. Cover, bring to the boil, then reduce the heat a little simmer for 15 mins till the pork is tender.
While the stew simmers, trim the dry ends off the cavolo nero. Finely shred it.
Stir the cavolo nero into the stew. Leave the lid off and simmer for 3 mins till the cavolo nero is just cooked and a little soft.
Taste the stew and add a pinch more salt and pepper if you think it needs it. Ladle into warm bowls and serve straight away.