Remove the heads from the prawns and reserve. Place the tails in a pan with the fish stock and the white wine, heat until the liquid starts to tremble and then poach the tails gently for 4 minutes. When cooked, drain them from the stock and allow to cool. Roughly chop up the heads from the prawns, and peel the asparagus tips if necessary. Cut each tip into 3 at a slight angle so that each piece is about 2.5 cm/1 inch long. When the prawns have cooled, shell them and remove and discard the dark line that runs the length of the tail. Add the shells to the chopped heads.
Heat the butter in a heavy-based saucepan until it starts to sizzle, add the shallot and garlic and fry gently for about 1 minute without browning. Add the chopped shells and heads and fry for a further minute. Add the brandy and reduce by half. Add the cooking stock and the thyme, bring to the boil and reduce by half. Strain the stock through muslin or a fine sieve into a clean saucepan, return to a high heat and reduce by half again. Pour in the cream, return to the boil and then add the asparagus. Continue boiling until the sauce has thickened and the asparagus has cooked. At this point, add the prawns, leave on a very low heat and allow the prawns to reheat and soak in the sauce.