You want the crisp exterior to break into a gloriously creamy interior. If you follow my grandma’s technique your croquettes will turn out perfectly every time.
Heat 4 Tbsp of olive oil in a small saucepan over a medium heat. Add the onion and cook over a low-medium heat until the onions become soft and transparent (about 6 minutes).
Stir the flour into the skillet over the onion and cook, stirring constantly, for about 3 minutes. It should be a bit compact and moist dough.
Meanwhile, in a small pot, bring the milk to a simmer and keep hot.
Gradually add the milk to the pan and 3 Tbsp of the brine liquid, stirring without stopping. This can take up to 10 minutes. If you get some lumps don´t worry, remove from the heat and in a separate bowl use a blender until the lumps disappeared. Pour it back in the saucepan and continue cooking.
Reduce the heat to low and fold in the chopped mussels. Cook gently, stirring for 3’ more. The mixture should be spongy and can be touched without sticking to your finger.
Spread the mixture onto a baking sheet, and allow to cool completely. Cover the mixture with cling film and refrigerate (use within 2 days).
Once it is completely cold, whisk the eggs in a shallow dish.
Stir the breadcrumbs in another shallow dish.
Scoop a tablespoons of cooled mixture, and shape with humid hands into medium size ovals. Take one croquette at a time and coat in beaten egg, then dip in the breadcrumb mixture. Set on a plate with parchment paper.
In an skillet Fry the croquettes, a few at a time, for 3-4 minutes or until crisp and golden, turning often. Drain on paper towels. Garnish with parsley if using.
Heat ½ cup of oil in a pan. Fry the croquettes in batches, turning once, until golden brown, about 2-4’total. Transfer to paper towels to drain. Add more oil as needed.
Serve warm and enjoy.
* To freeze: roll the uncooked croquettes in egg and breadcrumbs. Set separately on a tray and freeze. Once they are hard transfer them to zip bags. Take them from the freezer and fry directly, without thawing.