Peel the prawn, leaving the heads. Reserve the skins to make the broth.
In a saucepan add the sunflower oil and heat at medium heat. Then add the prawn skins and fry for. 2 minutes. Add the water, tomatoes, onion, garlic, merit and guajillo chilli. Bring this to a boil and lower the heat. Simmer for 15 minutes.
Remove the vegetables and chillies and blend until smooth. Set aside.
Remove the prawn skins and then add the vegetable and chilli sauce. Add the salt, oregano, carrots and potatoes. Cook until the carrots and potatoes are soft. it will take around 10 minutes.
Once the vegetables are soft, add the prawns. Cook for 5 minutes and then serve in a bowl. Add the prawns and some carrots and potatoes, then scatter some coriander and squeeze some lime. Enjoy!