Slice the mushrooms and garlic.
Heat the oil in a large frying pan. Add the garlic and stir-fry for a few seconds.
Add the mushrooms and stir for a minute. Add the chilli flakes, cover the pan with a lid, turn down the heat and cook for 5 minutes.
Add the lemon juice, let it sizzle, then pour over the stock. Put the cover on again and cook until the mushrooms have softened. This should take about 8 minutes.
Add the cavolo nero. Turn up the heat, cover and cook it for 3 minutes, then take off the cover and reduce the sauce.
Let it rest for 5 minutes under cover, then serve with chopped parsley and some extra chilli flakes, if you like.