I made this fumetto with the following ingredients:
1 kg of crystal prawns (any other prawns will do) - I suggest you buy these from the wet market as they are much cheaper, about $12 per kg.
1 celery stalk
1 whole garlic bulb
3 bay leaves
3 bay leaves
2 tsp of tomato concentrate
20 whole peppercorns
a bunch of parsley
fish bones (optional)
Peel and halve the onion, halve the whole garlic bulb, cut the celery into 4 pieces, peel the carrots and cut them lengthwise and then into 2. Peel the prawns and set aside the shells and heads. You can wash and freeze the meat.
Put the carrots, onions, celery, garlic on a large non-stick saucepan with a drip of olive oil, and fry them until golden on medium-high heat. Add the prawn shells and fish bones, continue to toss until the shells are nicely coloured. Add enough cold water to cover the content of the pan, then add the bay leaves, parsley and peppercorns and cover until almost boiling. Adjust the gas so that the liquid boils as slowly as possible, and let it boil stirring occasionally for a few hours (1-3) adding cold water when necessary. Finally add the tomato concentrate, continue to boil and reduce the liquid to about a third. Remove from the gas, let it rest and strain to obtain a clear liquid. The picture shows what is left after you separate the liquid from the ingredients, which you can then discard.
For convenience, you can freeze the stock by pouring it into little bags or small container of just the right size (about a cup for 2 people).
Alternatively, for a quick fumetto that you need on the day, follow the same procedure but only boil for about 30-40 minutes reducing it to a third.