Boil the prawns in a little water, then remove the shell and heads, which you will use for preparing the fumet. Boil 1 litre of water with 2 grains of black pepper and a bunch of parsley. Add salt, heads and shells. Reduce the fumet to half its volume and strain.
In a frying pan sauté the butter with the chopped onion, diced tomatoes and tomato paste.
Add white wine and simmer until evaporated.
Add the prawns cut into small pieces (leave a few whole prawns for the final garnish), and continue cooking for about 2 minutes.
Add the cornstarch dissolved in a bowl with a few spoonfuls of fumet, and add the remainder to the rest of the ingredients.
Leave to reduce, then pour in the cooking cream and cook on a high heat for approximately 2 minutes. Salt and pepper to taste.
Meanwhile cut the leek into very fine strips, dry well with kitchen towel and fry in hot extra virgin olive oil.
Once ready, drain well and place the fried leek on absorbent paper.
Toast the croutons cut into squares with a little butter in a non-stick pan. Keep warm.
Serve prawn cream in bowls or soup plates, using the whole prawns you have set aside and garnishing with the toasted croutons and fried leek.