Place the pork, prawns, 2 of the spring onions, the garlic, ginger and oyster sauce in a food processor and blend to a paste.
Take 1 of the wonton wrappers and place 1 teaspoon of the prawn and pork mixture in the centre. Dampen the edges of the wrapper with a little water and bring them up around the filling, enclosing it completely in a little bundle. Repeat with the remainder of the wrappers and prawn and pork mixture.
Bring the stock to the boil in a large saucepan, then reduce the heat, add the wontons and simmer for 4–5 minutes. Remove 1 of the wontons and check that it has become firm, which will indicate that it is cooked.
Add the spring greens to the pan and cook for 1 minute. Season the stock with the fish sauce.
Divide the soup between 4 bowls and serve with a few coriander leaves, a sprinkling of sesame seeds and a lime wedge.