Peel and devein the prawns. Remove the tomalley from the heads and put aside.
Brown the prawn heads and shells in olive oil and then add the onion, fennel fronds,marjoram, garlic and tomatoes.
Cook for 2-3 minutes. Add the water and and simmer for 15 minutes. Strain through a fine sieve.
Blend the tomalley with 200 ml of the fish stock, add back into the pot and cook over high heat to push all the impurities to the surface and to start the clarification process.
When the mixture starts to boil, lower the heat and simmer for about 1 hour.
Filter through a fine sieve and add 5 g of iota for each litre of liquid. Cool.