Heat the olive oil in a large saucepan, then fry the medium red onion and celery stalks gently for 10 minutes until softened, stirring frequently. Tip in the cumin and fry for another 1 minute.
Turn up the heat, then add the vegetable stock, canned plum tomatoes and canned chickpeas, plus a good grind of black pepper. Simmer for 8 minutes. Throw in broad frozen fava beans and lemon juice, cook for a further 2 minutes.
Season to taste, then top with a sprinkling of lemon zest and chopped coriander or parsley. Serve with flatbread.