The benchmark of Spanish tapas: crisp golden croquettes filled with an impossibly silky, slow-cooked béchamel studded with savory cured jamón Ibérico.
Melt butter in a heavy saucepan, add diced jamón and fry for 1 minute. Add flour and cook roux for 3 minutes. Slowly pour in warm milk while whisking vigorously. Simmer on low heat, stirring continuously for 15 minutes until thick, glossy, and pulling away from the pan. Season with nutmeg and pepper.
Spread the warm béchamel in a buttered shallow dish. Cover with clingfilm directly on the surface to prevent skin forming. Chill in fridge for at least 4 hours (or overnight).
Shape chilled dough into quenelles or cylinders using two spoons. Roll in flour, dip in beaten egg, and coat thoroughly in breadcrumbs.
Fry in hot oil at 180°C in small batches for 2-3 minutes until golden and crisp with a molten liquid centre. Drain on paper towels and serve warm.