In a large skillet heat 1 tablespoon of butter over medium heat to melt butter.Sauté the onion, stirring it until it turns translucent (2-3 minutes). Stir in the chicken and stir until it is heated through.Season with pepper, and parsley. Set aside.
2. Cooking bechamel
Heat the remaining butter over medium-low heat until it's melted. Add the flour all at once and whisk vigorously until it's combined with the butter. Cook stirring for two minutes.Add milk slowly while still whisking. Once all the milk has been added, keep stirring until the sauce starts to thicken. Season with salt to taste.Once it thickens enough that it starts lifting from the bottom of the pot remove itfrom the heat. Stir in the chicken until it's combined with the bechamel base. Stir until it cools down a bit.
3. Forming the croquettes
There are two ways to form them. You can put the chicken mixture in a pastry bag. Extrude it onto a greased tray or a tray lined with a Silpat. Or form them one by one, shaping them to the size of wine corks.Cover with plastic film and refrigerate for at least 3 hours.
4. Battering
Remove from the fridge. If you used a pastry bag, cut them into similar-sized croquettes.Dip the croquettes in the eggs, then cover them with breadcrumbs. Cover and freeze for 3 hours, or refrigerate overnight.
5. Frying
Heat oil over high heat (375 ºF [190 ºC]) in a small saucepan (there should be enough depth to cover the croquettes). Remove the croquettes from the freezer and immediately Fry them two by two until they turn golden brown.Rest on a paper towel to drain excess oil before serving.
6. Serving
They are best served freshly fried, but you can serve them within an hour at room temperature.
Video
Notes
You can make homemade breadcrumbs by pulverizing day-old bread in the food processor, or by grating dry bread. Panko give a more uniform texture and color to the croquettes.