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1
Melt the butter in a heavy saucepan over a medium heat. Add the flour and stir constantly for 2 minutes to cook out the raw taste. Begin adding the warmed milk in a slow, steady stream, whisking without pause. Continue cooking over a medium-low heat, stirring continuously, until the béchamel is extremely thick — far beyond a coating sauce. The decisive test: when a spoonful is lifted and dropped back into the pan, it should fall in a thick, flowing stream and then spread slowly. If it pours freely, it requires further cooking. If it holds a stiff, upright mound, it has been taken too far. This consistency is where the dish lives or dies.
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2
Remove from the heat. Fold in the finely chopped jamón serrano thoroughly, ensuring even distribution. Season with salt, white pepper, and nutmeg. Pour the mixture onto a lightly oiled sheet pan, spreading it to an even layer of roughly 2 cm depth. Press a sheet of cling film directly onto the surface to prevent a skin forming. Refrigerate for a minimum of 4 hours. The mixture is ready when it is fully set, firm to the touch, and lifts cleanly from the pan without slumping.
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3
Set up three shallow dishes in sequence: plain flour, beaten egg, fine breadcrumbs. Working with cold, damp hands, portion the chilled béchamel and roll into cylinders of uniform size. Pass each cylinder through flour, then egg, then breadcrumbs. Return immediately to egg and breadcrumbs for a second coat. The finished croquetas should show an even, unbroken crust with no patches of bare filling visible.
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4
Heat the oil in a deep, heavy pan to 180°C. Fry the croquetas in small batches, turning as needed, until the exterior is a deep, uniform golden brown. Do not crowd the pan; the oil temperature must remain stable. Lift one croqueta from the oil: the crust should be dry-sounding when tapped lightly and show no pale or greasy patches. Drain on absorbent paper and serve immediately.