Begin by making the tomato element of the sauce. Cut the tomatoes into quarters and place on a baking sheet, skin side down. Drizzle with olive oil and generously season with both salt and pepper. Place in the oven and cook for 30-45 minutes until the tomatoes have dried out slightly and the skin is blistered. Remove from the oven and allow to cool.
While the tomatoes are cooling, cut a lemon in half and place in a large non-stick pan, flesh side down, while the pan heats. Continue to cook over high heat until the lemon flesh touching the pan begins to burn. Remove from the pan and set aside, then decrease the heat to medium. Pour in a splash of oil, followed by the Old Bay or Cajun seasoning. Heat for 30 seconds or so, stirring constantly, then add the prawns to the pan, followed by the chopped parsley. Cook for two minutes on medium heat, then turn over and cook for another minute or two until cooked through. Remove from the heat and squeeze the juice form the burnt lemon over the prawns. Transfer the cooked prawns to a plate for cooling and save the residual contents of the pan.
Once the prawns have cooled, remove the shells and cut half into small chunks, while keeping half of the prawns whole. Return to a medium mixing bowl.
Once the tomatoes have cooled, add to a blender (or chop finely) and pass through a sieve. Mix this with the aioli, adding a tablespoon at a time and tasting often to find the right amount. The colour should be a light, pastel pink. Then add the Worcestershire sauce, Tabasco, a tablespoon or two of the leftover juices from the prawn pan, and salt and pepper if necessary.
Spoon the sauce over the prawns, adding just enough to prevent the prawn cocktail from becoming overly watery. Mix as you go and add more as necessary.
To serve, half fill four glasses or bowls with shredded iceberg lettuce, followed by a generous scoop of the prawn cocktail, topped with a small smattering of smoked paprika and an optional spoonful of herring roe or caviar. Serve with buttered brown bread and ideally an ice cold white wine or a glass of champagne.