Peel and finely chop the onion and carrot. Peel and grate the garlic. Thickly slice the mushrooms.
Crumble the stock cube into a jug. Add the dried porcini mushrooms. Pour over 400ml boiling water. Stir well and leave to infuse and soften.
Set a large pan on a medium heat. When warm, drizzle in 1 tbsp olive oil. Add the chicken and fry for 5 mins till golden. Add the onion and carrot. Cook for a further 5 mins till the onion has softened. Stir every now and then.
Add the garlic and mushrooms to the pan and cook for another 5 mins till the mushrooms have softened slightly.
Pour in 1 tbsp balsamic vinegar, the porcini infused stock and the chopped tomatoes. Stir everything together. Pop on a lid. Simmer on a gentle heat for 20 mins.
While the chicken is simmering, finely shred the cavolo nero, discarding any tough woody ends. After 20 mins, taste the chicken and add salt and pepper to taste. Drop in the gnocchi and stir. Add the cavolo nero and stir. Pop the lid back on and cook for 5 mins till the cavolo nero has wilted and the gnocchi is cooked.
Spoon the chicken and gnocchi dumplings into a couple of warm bowls. Drizzle 1 tsp oil over each bowl and serve.