Heat a little oil in a large saucepan and cook the onion for around 5 minutes, until softened. Add the garlic and leeks and cook for another minute. Add the harissa paste, chopped tomatoes and vegetable stock, and simmer for 20 minutes.
Add the cannellini beans and simmer for another couple of minutes. Taste for seasoning and add the lemon juice - between half and a whole lemon. Taste before adding the whole lot because too much and it tastes like washing up liquid! Stir in the chopped parsley and serve, with crusty bread to mop up the sauce.