To make the bisque, heat two tablespoons of vegetable oil in a large pan and fry the carrots, leek and celery for a few minutes to soften.
Add the chilli and ginger and stir in the tomato purée, thyme, bay and tarragon and continue to cook for 1-2 minutes.
Add the prawn shells and heads and cook in the pan until golden-brown. Add the fish bones and then pour in the cognac and white wine and stir to deglaze.
Pour in enough water to cover the bones, bring to the boil and then reduce the heat and allow to simmer gently for an hour.
Allow the bisque to cool slightly, then blend using a hand blender. Strain into a clean pan. Cook to reduce the volume of liquid for a further 30 minutes, or until reduced by a third.
Stir in the cream to taste and season to taste with salt and freshly ground black pepper.
To make the tempura, mix the flour, cornflour and salt in a large bowl. Gradually whisk in the sparkling water to form a smooth batter. Set aside for 20-30 minutes.
Heat a deep-fat fryer to 180C/350F. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Dip the prawns into the batter and then deep fry in the hot oil for 1-2 minutes, or until golden-brown and cooked through.
To serve, arrange the prawns on serving plates and garnish with slices of spring onion. Spoon the bisque into a serving dishes alongside and serve scattered with chopped coriander.