Place the milk, butter and flour in a medium pan, season well, then slowly bring to the boil, whisking with a balloon whisk until the mixture boils and thickens. Reduce the heat and cook for 1 minute, stirring all the time until you have a thick, smooth sauce. Remove from the heat, stir in the puree, zest and juice, parsley and adjust seasoning to taste. Transfer to a bowl to cool. When the mixture is cold, stir in the prawns. To shape the croquettes: sprinkle the breadcrumbs on a plate, beat the eggs and pour into a shallow dish, and sprinkle flour onto another plate. Use two dessertspoons to scoop and divide the mixture into about 16 pieces. Use wet hands to shape each piece into little oval balls, then dip each first in flour, then egg, then breadcrumbs. Place on a plate and chill until ready to cook. Heat the oil to 180°C in a deep pan or fryer. Add the croquettes a few at a time and cook for about 3 mins, turning them regularly until they are golden brown. Drain on kitchen paper. Serve hot with wedges of lemon.