Bring a large saucepan of salted water to the boil. Add the garlic and simmer for 4–5 minutes.
Add the spinach and stir for about 30 seconds until wilted; then drain into a colander.
Once the spinach has cooled slightly (don’t blitz it when it’s boiling hot) put both the spinach and garlic in a high-speed blender, along with the basil, chilli flakes (if using), lemon zest, olive oil and a large pinch of salt. Blitz until smooth; taste to check the seasoning and adjust as needed.
Pour the sauce into the saucepan, followed by the beans, cavolo nero and a splash of water (about 2 tablespoons). Cover with a lid and cook over medium heat for 5 minutes, until the cavolo nero has wilted. Season to taste with salt and pepper.
Serve immediately with plenty of toast and an extra drizzle of olive oil.
