Finely chop the Onion and the Jamón Serrano; set aside.
In a medium saucepan over medium heat, melt the Unsalted Butter together with a splash of Olive Oil. Add the chopped Onion and sweat gently until translucent and fragrant, about 5–6 minutes.
Stir in the Plain Flour and cook, stirring constantly, for 2 minutes to make a light roux without browning.
Gradually whisk in warmed Whole Milk a little at a time until the mixture is smooth, then simmer gently, whisking, until the béchamel is thick and coats the back of a spoon (about 5–8 minutes).
Stir in the chopped Jamón Serrano and season with a pinch of Ground Nutmeg, freshly ground Black Pepper and Fine Salt to taste; cook another 1–2 minutes to meld the flavors.
Spread the mixture into a shallow dish, cover with cling film (press film onto the surface to prevent a skin) and chill in the fridge until firm, at least 1–2 hours.
When the filling is firm, shape it into small logs or balls using wet hands. Prepare a dredging station with a shallow plate of Plain Flour, a bowl with beaten Eggs, and a plate of Fine Breadcrumbs.
Coat each croqueta first in Plain Flour, then dip into the beaten Eggs, and finally roll in the Fine Breadcrumbs, pressing gently so the crumbs adhere evenly.
Heat enough Mild Olive Oil in a deep frying pan to come about 1–1.5 cm up the side and warm to medium-high (about 170–180°C if using a thermometer). Fry the croquetas in batches for 2–3 minutes, turning so they brown evenly, until golden and crisp.
Remove with a slotted spoon and drain on paper towels. Serve hot as they are or with a squeeze of lemon or your favorite dipping sauce.
