2Heat the milk and cream until just about to boil.
3Remove from heat, add the jamón, and cover the pot to infuse the liquid with flavor.
4Crack eggs and set the yolks aside. Fry the egg whites until golden and crispy.
5Finely dice the onion.
6Melt butter with olive oil in a pan and sauté the onion with a pinch of salt and pepper over medium heat for 15-20 minutes until all moisture is gone.
7Add flour and stir to combine with the fat, forming a roux. Cook the roux for another 10 minutes over medium heat.
8Gradually add the warm milk mixture in three or four additions, stirring continuously with a spatula to avoid lumps.
9Once all the milk is incorporated, add the chopped fried egg whites and the reserved egg yolks.
10Chill the mixture in the fridge for 12-24 hours.
11Shape the croquetas with two spoons.
12Freeze the shaped croquetas for 4-6 hours.
13Coat the frozen croquetas in beaten egg and breadcrumbs (panko).
14Fry the croquetas at 180°C (356°F) for about 2-3 minutes in small batches.