Wash the prawns and remove their heads but leave the shells.
Cut the white parts of the spring onion and crush lightly. Cut the green parts into 5cm strips and set aside.
Heat oil in a pan. Saute the white part of the spring onions gently until fragrant. Add the prawns and toss until they begin to turn pink.
Add the caramel sauce, fish sauce, salt and sugar. Reduce the sauce until the prawns are coated in a sticky gravy.
If the mixture starts to burn, add a little water until a dense sauce is formed. Add the green spring onion and stir until wilted.
To make the Caramel Sauce: Put sugar in a small saucepan with 2 tsp of water. Gently melt the sugar in the water. Continue to cook until the sugar caramelises, turning into the colour of strong black tea.
Remove from heat and stir in remaining water, a little at a time. Swirl the pan so that the caramel dissolves in the water.
When the caramel and water have combined, pour into a jar and store in the refrigerator for future use.