Heat the oil in a frying pan, briefly sauté the shallot, stir fry the seafood for approx. 5 mins., press out the liquid, chop finely.
Heat the oil in a pan. Add the flour and cook, stirring with a whisk; the flour must not take on any colour. Remove the pan from the heat. Pour in the milk, bring to the boil while stirring, reduce the heat and simmer for approx. 10 mins., stirring occasionally.
Add the seafood, lemon zest and juice, mix and season. Transfer the mixture to a piping bag with a smooth nozzle (approx. 1 cm in diameter), pipe two rolls, cut into approx. 16 croquettes.
Place the egg and white flour in a deep bowl. Toss the croquettes in the egg & flour mixture and then in the breadcrumbs, firmly pressing on the crumb coating.
Fill a pot to 1/3 with oil and heat it to approx. 180°C. Deep-fry the croquettes in batches for approx. 5 mins. until golden yellow, turning occasionally. Remove and drain on paper towels.