In a food processor place tomatoes. Add a pinch of coarse salt and purée. Strain the purée through a cheesecloth-lined sieve set over a stainless steel or glass bowl to collect the tomato water. Allow to drain a few hours or overnight in refrigerator.
In a saucepan place strained tomato water and bring to a boil on medium-high heat. Reduce heat and adjust seasoning. Keep warm.
In a non-stick frying pan, heat the vegetable oil over medium-high heat. Season prawns with salt and pepper. When the pan is hot, add the prawns and quickly sauté. Remove from heat and add butter.
To serve place two prawns in the centre of each bowl with a little tomato concasse, ladle in warm soup and serve immediately.
For a deep rich colour and flavour, add a pinch of Saffron when heating the tomato water.