Heat the oil in a large saucepan. Add the onion and garlic and cook over a medium heat until the onion and garlic are soft but not coloured, about 4-5 minutes. Add the fennel seeds, tomatoes, water, chilli and lemon zest and cook for 10 minutes. Season with black pepper.
Add the mussels and spinach into the tomato sauce. Cover and cook for 4-5 minutes, shaking the saucepan a few times until the mussels have opened. Discard any that have not opened. Taste and season with a little salt and freshly ground black pepper.