Melt the butter in a large saucepan over a medium heat, add the onion and cook gently for 10 minutes to soften the onion. Add the chorizo to the pan and cook for a further 5 minutes, then add the cooked chicken to the pan and mix well.
Reduce the heat to low, add the flour and the nutmeg to the pan and cook the flour gently for 5 minutes. Stir constantly as you want the flour to cook but not burn.
Gradually add the chicken stock to the pan and stir continuously to avoid the sauce becoming lumpy. The sauce will start to thicken, you want a smooth fairly thick sauce. Once you've achieved this take it off the heat, stir through the parsley and season to taste.
Remove the pan from the heat, pour the sauce into a flat dish and leave to cool. Once cool place the mixture in the fridge for an hour to become cold and thicken.
Taking a heaped tablespoon of the mixture, shape into a quenelle. [At this stage you can freeze the croquettes. Place each individual quenelle onto a lined baking sheet and open freeze for 30 minutes. Remove from the freezer and freeze as normal in a bag or box. When ready to cook remove from the freezer and defrost in the fridge overnight.]
Dip each quenelle into the seasoned flour, then into the beaten egg and finally into the panko breadcrumbs. Place on a plate and once all croquettes have been bread-crumbed, pop the plate back into the fridge for 30 minutes.
Pour the sunflower oil in a medium sized saucepan and heat to 180C. Place each croquette onto a slotted spoon and lower gently into the hot oil. Do not over-crowd the pan, cook the croquettes in batches, turning once to ensure an even cooking on both sides. Watch the temperature of the oil and lower if you think the croquettes are cooking too quickly.
When golden brown remove from the pan onto a plate lined with kitchen paper to absorb any excess oil. Serve immediately when still warm.