Preheat the oven to 180C/170C fan/gas mark 4. Put the porcini into a bowl and cover with boiling water. Leave to soak for 20 minutes.
Heat the oil in a large frying or sauté pan, and sauté the onion until it is soft and golden. Add the garlic and cook for a further two minutes.
Pull the leafy bit off the cavolo nero and discard the firm central ribs. Tear the leafy bits into pieces. Cook the cavolo nero for five minutes in boiling water, then drain really well.
When the cabbage has cooled enough to handle, squeeze it with your hands to remove excess water.
Drain the mushrooms (keep the soaking liquor for a mushroom risotto – it’s delicious). Add to the onion in the pan along with the butter, parsley and some seasoning, and sauté for a minute. Add the cavolo nero and gently stir everything together until mixed.
Scrape into a large bowl and add one third of the breadcrumbs, the eggs, nutmeg, all but 1 tbsp of the parmesan, and season.
Butter or oil and base-line a 20cm springform cake tin and sprinkle half of the remaining breadcrumbs on the bottom. (If you don’t want to turn the tortino out you can just bake it in a buttered oven-proof dish.)
Pour the mixture into the tin and sprinkle with the remaining breadcrumbs and parmesan. Bake in the oven until just set – about 20 minutes.
Leave to rest for 10 minutes, then run a knife round the edge of the tortino. Remove the ring and carefully remove the base and paper before putting the tortino on to a plate. Cut into four wedges and serve.