Peel the cooked shrimps. Keep the bodies in a bowl. Put the heads, tails and scales in a pot. Fill the pot with water until almost covering the content. Add a whole tomato, slightly notched. Bring to a boil, then turn off the heat.
Mix roughly with a hand mixer, until getting a big crumb consistency. Pass the mixture through a sieve, pressing it with a tablespoon to squeeze out all the juice.
Let the liquid cool down completely (I personally put it in the fridge after cooling down a bit, since serving it cold is delicious and refreshing). Discard the rest.
Chop the red onion thinly in a bowl. Add some big salt, mix, and rinse thouroughly to soften the taste. Add the juice of one lemon
Chop the two tomatoes and the green pepper in small dice.
Mix the shrimps, the red onion, the tomatoes and the green pepper in a large bowl.
Add the cooled down liquid until almost covering the mixture.
Add aproximately 1/4 of a teaspoon of mustard, and two tablespoons of olive oil.
Chop the cilantro thinly and add to the bowl.
Season to taste with salt, pepper, and extra lemon if needed.
Mix thouroughly.