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Melt half the butter in a small pan over a medium heat and fry the red onions for 5-8 mins until soft but not golden. Remove to a small bowl, leaving any residual butter in the pan. Set aside. Add the remaining butter to the pan and reduce the heat to medium-low. Once melted, whisk in the flour. Cook for 1 min, then gradually whisk in the milk, a little at a time. When all the milk has been added and you have a glossy, smooth paste, cook for another 1 min, stirring continuously.
Remove from the heat and stir in the cooked red onions, lemon zest, parsley and spring onions. Leave to cool for a few minutes, then stir in the crabmeat and season lightly. Transfer to a bowl, cover and chill for at least 2 hrs or overnight.
When the crab mixture is cold and has firmed up slightly, tip the beaten egg into a shallow bowl and pour the breadcrumbs onto a plate. Working quickly, shape 2 tbsp of the crab mixture into a small patty (see tip, below). Coat this in the egg, then the breadcrumbs, ensuring the croquette is completely covered. Transfer to a plate or board, then repeat with the rest of the crab mixture, beaten egg and the breadcrumbs – you should get about eight croquettes in total.
Heat a 2-3cm depth of vegetable oil in a frying pan to 180C, ensuring the pan is no more than a third full. You can also test the temperature by dropping in a pinch of any residual crab mixture – it should sizzle and turn golden within a couple of minutes. Fry the croquettes in batches (not overcrowding the pan) for 1-2 mins on each side until golden and piping hot through the middle. Remove to a sheet of kitchen paper using a fish slice or slotted spoon and leave to drain while you fry the rest of the croquettes. To make the sriracha mayo, simply combine 1 tbsp sriracha with the mayonnaise, then transfer to a small bowl. Taste and add more sriracha if you prefer.
Serve immediately, or if you’re taking the croquettes to the beach, leave to cool completely, then transfer to an airtight container. Spoon the sauce into a sealed jar. Pack both in a cool bag to serve at your destination. Will keep chilled for up to two days.