Heat oil and fry the paste till fragrant.add prawn stock, simmer and bring to a boil. Stir in the mashed potatoes and thicken with the cornstarch and season with sugar and salt and pepper.
Serve with fried tofu, slices of hard boiled egg, prawns, beansprouts, potatoes and a squeeze of lime juice.
To make prawn stock: Bring 2.5 litre water to a boil and pour in the prawns. When prawns are cooked, dish them out. Shell the prawns and keep aside for the noodles. Return the prawn shells to the pot to make prawn stock. Continue boiling the stock for another 30 minutes. Discard the shells.