For the tom yum paste, place all of the paste ingredients into a food processor and blend until smooth.
Heat a dash of vegetable oil in a pan and gently fry the paste for 4-5 minutes, strring frequently, or until it starts to become aromatic.
For the soup, add the milk and stock to the pan with the paste and stir well. Bring to the boil, then reduce to a simmer and cook for 10 minutes.
Place the beansprouts, pepper and onion into the bottom of four serving bowls.
Stir the aubergine and peas into the soup to warm through, then add the prawns and simmer until pink and cooked through. Ladle the soup into four serving bowls (there should be three prawns per serving). Serve with a squeeze of lime and garnish with the reserved coriander leaves.