Melt the butter in a medium sauté pan over medium heat. Add the onions and cook until they become translucent, about 5 minutes. Slowly stir in 1 ½ cups of the flour and continue stirring vigorously until well combined. Cook for about 3 minutes, until mixture becomes frothy and golden in color. Pour the milk into the mixture, stirring continuously, and continue to cook for about 2 minutes, until you have a thick béchamel sauce. Season to taste with ½ teaspoon of salt and nutmeg.

Add the chicken and stir with a wooden spoon until well combined. Cook for another 2 minutes, until you have a thick mixture you can mold in your hands. Carefully pick up a bit of the mixture and try to ball it with your hands. It shouldn’t be too sticky. If it does stick to your hands, allow the mixture to cook a little longer. Spread the mixture on a cookie sheet to allow it to cool.

Once it’s cool enough to handle, take a spoonful and roll it in your hands to make a small cylinder, about the size of a wine cork. Roll the cylinder in the remaining 1 cup of flour, then in the eggs, and then in the breadcrumbs, and set aside on another cookie sheet. Repeat with the remaining mixture. You should end up with about 36 croquetas.

In a small deep-frying pan, heat the olive oil to 375 F. Working in batches, add the croquetas, making sure they are covered completely in oil. Fry until golden, about 1 minute, then transfer to a tray lined with paper towel to drain. Be sure to allow the oil to return to 375 F between batches.

Season with salt and serve hot.