Bring 2 separate pans of salted water to the boil. Cook the broad beans for 3 minutes in one pan and cook the peas in the other pan for 2 minutes. Drain and set aside.
Peel the prawns, keeping the tails intact. Remove the black spine using the tip of a sharp knife. Place the prawns in a heatproof bowl.
Peel and finely slice the carrots and shallot.
In a frying pan with a splash of olive oil, gently cook the shallot and the carrots. Add the water, white wine, white balsamic vinegar and sugar. Add 2 sprigs of tarragon and a pinch of salt. Bring to the boil and pour this mixture over the prawns. Cover the bowl with foil and allow the prawns to poach in the liquid for about 5 minutes.
Place the broad beans and peas into a soup bowl. Add the prawns and pour over some of the stock. Garnish with fresh herbs and serve immediately.
«The heat of the stock is normally sufficient to cook the prawns but feel free to reheat over a low flame if they are not cooked through.»