Tip the dried porcini into a jug and crumble in the vegetable stock cube. Pour in 400ml boiling water and stir well till the stock cube has dissolved. Set the stock aside for 15 mins.
Meanwhile, peel the onions and finely slice them. Peel the garlic and crush or finely chop it. Trim any woody ends from the cavolo nero, and finely shred the leaves. Pick the thyme leaves off their sprigs (if you have a fine mesh sieve you can push the woody stalks through the holes in the mesh – the leaves will gather in the sieve).
Pour 1 tbsp oil into a large pan and bring to a medium heat. Add the chopped onions and garlic. Sprinkle in the thyme leaves and add a pinch of salt and pepper. Cook for 5–6 mins, stirring occasionally, till softened.
Stir the shredded cavolo nero leaves into the pan. Pour in the soaked porcini mushrooms and their stock. Bring to a simmer and cook for 2 mins, till the cavolo nero has just started to wilt.
Crumble in half the pack of the Perl Las cheese (see our tip below for what to do with the remaining half). Tumble the gnocchi into the pan and stir everything together, stirring till the cheese has melted into the sauce and the gnocchi is coated. Cook for a further 4–5 mins, till the gnocchi is plump and tender.
Tip the rocket into a large bowl. Squeeze over the juice from half the lemon and sprinkle in a little salt and pepper. Toss together till dressed.
Spoon the gnocchi and sauce into a couple of warm bowls and serve with the rocket on the side.