Place a large saucepan on the heat, and add 2 tablespoons of mild olive oil and the shallots. Cook for 2 minutes over a medium heat, then add the garlic and cook for a further 2 minutes. Add the cumin and chickpeas, cover with 400ml of water and simmer for 5 minutes.
Meanwhile, place the shells, tails and heads from the prawns in a medium sized saucepan and pour in the mild olive oil. Turn up the heat, smash up the shells to release their colour and flavour and cook for 2 minutes, then remove from the heat and pour the mixture through a sieve, retaining the flavoured oil and discarding the shells.
Remove the pan of chickpeas from the heat. Drain the chickpeas in a colander, keeping the cooking liquid, and place them in a food processor. Blend together, adding the reserved cooking liquid a little at a time, until smooth. Gradually pour in the extra virgin olive oil and continue to process until you have a smooth purée. Season to taste with salt and pepper.
Put a large frying pan on the heat and add some of the prawn oil. When hot, add the prawns and sauté over a medium heat for 3 minutes, turning them over, until they have turned pink and are cooked through. Remove the prawns from the heat and squeeze over the lemon juice.
Drizzle the sourdough slices with a little extra virgin olive oil and place on a hot griddle for 1 minute on each side until well toasted.
Spoon the chickpea purée into the centre of each plate and top with the prawns. Garnish with the chopped coriander cress or coriander and a piece of sourdough toast to the side.