Peel and halve the onions through the root, then slice thinly lengthways. Heat 2 tbsp of the oil in a large pan, add half the onions and fry until just starting to colour. Add the mustard and cumin seeds and fry until nicely browned.
Add vegetables and curry paste and stir until well coated. Pour in the stock and bring to the boil. Reduce the heat, partly cover and simmer for 30 mins, until the vegetables are tender. Meanwhile, heat the remaining tbsp of oil in a small pan, add the onions and fry quickly until crisp and browned. Tip onto kitchen paper.
Purée the soup in batches, then return to the pan and stir in most of the yogurt. Taste and add salt, if necessary. Reheat gently, then ladle into bowls and top each with a spoonful of yogurt, some fried onions and a scattering of roughly chopped coriander or parsley.