Melt butter over medium heat. Add the onions and cook until they become translucent, about 5 minutes. Stir in 1 1/2 cups of the flour and stir constantly. Cook the roux for 5 minutes until the flour is cooked through and golden brown. Pour the milk into the flour mixture and cook, stirring constantly, for about 2 minutes, until you have a thick sauce.
Add the chicken or other protein/veggie, and sprinkle in 1/2 teaspoon of salt and the nutmeg. Cook for another 2 minutes and cool a little; it should be very thick. Try to make a little ball in your hands; it shouldn't stick too much. If it does stick, return to pan and cook a little longer. Spread the mixture on a baking sheet and cool.
Take a a golf-ball sized ball of the mixture and roll in your hands to make a cylinder. Roll in the remaining 1 cup flour, then in the eggs, and then in the breadcrumbs and set aside to dry on another cookie sheet. Repeat with the remaining mixture.
In a small deep frying pan, heat the olive oil to 375 degrees. Working in small batches, add the croquetas, making sure they are covered completely in oil. Fry until golden, about 1 minute, then transfer to a paper towel-lined rack to drain. Be sure to allow the oil to return to 375 degrees F on a thermometer between batches. Season with salt and serve hot.