Melt the butter in a saucepan on a medium heat.
When it’s bubbling, add the garlic, fennel seeds and chilli flakes, and fry, stirring, for 2 to 3 minutes, until the garlic smell changes from raw to cooked.
Add the broccoli, cavolo nero, salt and 250ml of water, stir, cover, turn down the heat to medium-low and cook, stirring every few minutes, for 8 to 10 minutes, until the greens have wilted and become tender.
Scrape all the contents of the cavolo nero pan into a blender or food processor, add the miso and olive oil, and blend to a smooth sauce.
Rinse out the greens pot, fill with water and bring to a boil.
Cook the pasta according to the packet instructions and, when it’s got a minute to go, scoop out a mugful of the starchy cooking water.
Drain the pasta, return it to the pot, add the sauce and toss with around 6 to 8 tablespoons of the cooking water to get it to a consistency you like.
Taste and add salt, if need be.
Spoon out on to a serving platter and drizzle over with chilli oil or extra virgin olive oil.
