Melt the butter in a medium saucepan over medium-low heat. Add the onion and serrano ham, cooking for about 3 minutes. Season with nutmeg, salt, and white pepper.

Sprinkle in the flour and cook, stirring constantly, for about 5 minutes, until the mixture is fully coated.

In a separate saucepan, heat the milk almost to a boiling point. Remove from the heat and set aside. Gradually whisk in the hot milk to the ham mixture, one ladle at a time. This will take about 4 to 5 minutes.

Reduce the heat to low, stir in the grated Manchego cheese, and continue to stir for 25 to 30 minutes (making sure the béchamel sauce doesn’t stick to the bottom of the pan), or until the sauce is thick and silky. Taste and adjust the seasoning if needed.

Transfer the filling to a parchment-lined loaf pan and spread it out evenly. Allow the mixture to cool to room temperature. Press a piece of cling wrap directly onto the surface to prevent a skin from forming. Refrigerate for at least 4 hours, or preferably overnight.

When ready to cook, remove the filling from the refrigerator and place it on a lightly floured surface. Carefully peel away the parchment paper. Sprinkle a little more flour on top, then use a knife to cut the béchamel into 3 strips and then 18 to 24 (2-inch) squares (depending on the size of your lo

Dust your hands with flour and roll the squares into balls between your hands.

In a bowl, whisk the eggs and set aside. In another bowl, mix the panko breadcrumbs. Dip each croquette into the egg wash, then coat in the panko breadcrumbs. Transfer the shaped croquettes back to the refrigerator and chill for 15 to 20 minutes before frying.

Heat the olive oil in a large deep pan until it reaches 350°F. Working in batches, fry the croquettes until golden brown, about 2 minutes. Transfer to a cooling rack to drain.