Heat the oil and butter in a large pan. Add the onion, garlic, mustard and paprika and fry for about 5 mins, until softened.
Sprinkle in the flour and stir until absorbed. Remove from the heat and add the stock in batches, stirring after each addition, until smooth. Return to the heat and add the chopped potatoes.
Bring to the boil then reduce the heat and simmer, stirring occasionally, for 10-15 mins until the potatoes are tender. Add the corn kernels and simmer for a further 5 mins.
Pour in the milk and cream, then add the prawns. Heat for about 5 mins, without boiling, until the prawns are pink and cooked through.
Stir through the parsley. Season to taste and serve.