This Swedish recipe makes slightly over a litre of soup, but scales up well. I've given fairly precise quantities, but you don't have to be that exact. And in the absence of fish stock, I've also made it with chicken or vegetable stock instead, with perfectly good results.
600g cooked but unpeeled prawns
about 60g butter
1 medium sized onion
a small piece of celeriac (about 30g)
1 dsp. tomato puree
60cl fish stock
60g plain flour
40cl full fat milk
a few shakes of cayenne pepper
Garnish: fresh dill
Peel and finely chop the onion and celeriac. Peel the prawns and saute the shards (heads, tails, body shell - the lot) gently in the butter in a deepish pan for about 5 or 6 minutes, stirring from time to time to make sure they don't burn and stick to the pan.
Add the chopped veg to the pan, then the tomato puree, and stir. Pour in the fish stock, stir again, and simmer for about 20 minutes.Sieve to remove the prawn shards and veg, draining the liquid into a bowl, and discard the shards etc. Return the liquid to the pan. Mix the flour with a little of the milk in a ramekin, or something similar, and then stir the mixture into the prawn liquid. Add the rest of the milk, seasoning to taste, and the cayenne pepper, and simmer for a further 5 minutes.
Add the prawns and serve, garnished with the fresh dill.