Cut the cavolo nero off the tough rib running up the centre of the leaf and then chop up roughly. Place in a sink of cold water and wash thoroughly (no one wants caterpillars in their dinner!). Place in a colander and leave to drain.
Put a large frying pan on medium heat and add the olive oil.
Slice up your onion and add to the pan, cook for a few minutes while you finely chop up the garlic. Once the onion has softened a little add the garlic to the pan and stir. As that cooks, dice up your mushrooms and tomato and add these to the pan as you go.
Add the drained cavolo nero, chilli flakes, salt and pepper and give everything a good stir. Drizzle a little water into the pan and cover with a lid to help the cavolo nero wilt down. Cook on a low heat for about 15 minutes. If it starts looking a little dry, you can add a little extra water.