Croquetas

Method

1. Melt 15 g of butter in a small heavy saucepan over low heat. Add 20 g of very finely chopped onion and sweat gently for 5-6 minutes, stirring, until soft and translucent but with no colour at all.
2. Stir in 20 g of plain flour and cook it in the butter for 2 minutes over low heat, stirring constantly, until it smells biscuity but stays pale. This is the roux.
3. Add 150 ml of whole milk a splash at a time, beating hard with a wooden spoon or whisk so each addition is fully absorbed before the next. Once all the milk is in, cook over low heat for 10-12 minutes, stirring almost without stopping, until the mass is glossy, thick enough to hold the mark of the spoon and comes away cleanly from the sides of the pan.
4. Off the heat, stir in 40 g of finely chopped jamón serrano, a small grating of nutmeg and a good grind of black pepper. Taste and add salt only if needed, as the ham is already salty. Spread the mixture 2 cm deep in a shallow dish, press clingfilm onto the surface and chill at least 3 hours, or overnight, until completely firm.