Heat 2 tablespoons of vegetable oil in a large pan and fry carrots, leek, and celery until soft.
Stir in chilli, ginger, tomato purée, thyme, bay, and tarragon and cook for 2 minutes.
Add prawn shells, heads, and fish bones to the pan and fry until golden-brown.
Add cognac and white wine to deglaze the pan.
Cover bones with water, bring to boil, then simmer gently for one hour.
Blend the mixture, strain into a clean pan, and reduce by a third over 30 minutes.
Stir in cream and season with salt and pepper.
Whisk flour, cornflour, salt, and sparkling water into a smooth batter and rest for 30 minutes.
Heat a deep-fat fryer to 180°C.
Dip prawns into batter and deep fry for 2 minutes until golden.
Garnish prawns with spring onions and serve with the bisque and chopped coriander.