Heat the butter and olive oil in a large non-stick pan over medium heat, then add the minced onion and sauté until translucent and soft, about 5 minutes.

Stir in the chopped Serrano ham and cook for another 2 minutes to release the savory fat and flavor into the oil mixture.

Add the flour to the pan and cook for 3-4 minutes, stirring constantly, to toast the flour lightly and remove the raw taste.

Gradually pour in the warm milk while whisking vigorously to prevent lumps, cooking and stirring continuously until the mixture thickens into a smooth, dense dough that pulls away cleanly from the sides of the pan.

Season the dough with nutmeg and black pepper (taste before adding salt, as the ham is naturally salty), then transfer to a shallow dish, cover with plastic wrap pressing directly onto the surface, and refrigerate for at least 2 hours until firm.

Once the dough is chilled and set, scoop small portions and shape them into small logs or oval cylinders with your hands.

Dip each croquette into the beaten eggs, ensuring it is fully coated, then roll it in the breadcrumbs until an even crust forms.

Heat about 2 inches of vegetable oil in a deep frying pan to 350°F (175°C) and fry the croquetas in batches for 2-3 minutes until they are golden brown and crispy.

Remove the croquetas with a slotted spoon and drain on paper towels before serving warm with a side of aioli or just as they are.