2) Meanwhile, heat the olive oil in a large pan or Dutch oven. Add the leeks and cook for 10 minutes over a medium-low heat, or until the leeks are tender but not browned. Add the garlic and cook 1 more minute. Add the cayenne pepper and prawns and cook over a medium to low heat for 3 minutes, stirring occasionally. Add the Cognac and cook for 1 minute, then the sherry and cook for 3 minutes longer.
3) Transfer the prawn and leek mixture to a food processor fitted with a steel blade and process until coarsely pureed.
4) In the same pan, melt the butter. Add the flour and cook over a medium-low heat for 1 minute, stirring with a wooden spoon. Add the single cream and cook, stirring with a whisk, until thickened, about 3 minutes. Stir in the pureed prawns, the stock, tomato puree, salt and pepper and heat gently until hot but not boiling. Season, to taste, and serve hot.