Thinly slice the onion and garlic. Roughly chop the broccoli.
Heat the oil in a saucepan, then gently fry the onion and garlic until soft. Add the broccoli, bay leaf and stock, bring the boil, then cook for 15 minutes, until the broccoli has softened.
Add the coriander leaves and blend in a liquidiser or with a stick-blender until smooth and silky (or slightly textured if preferred).
In a frying pan, heat the oil and fry the mustard seeds and cumin seeds for a minute, until they start sizzling and popping. Add the spices and stir for a few seconds, until aromatic.
Add the tadka mixture (including the oil) to the soup and stir. Reheat if necessary.
Serve with a squeeze of lime juice and ground pepper.