1. In a mixing bowl, combine the shredded chicken, bread crumbs, milk, beaten egg, chopped onion, flour, salt, and black pepper. Mix well to form a dough.
2. Shape the mixture into small patties or balls.
3. In a frying pan, heat the butter and oil over medium heat.
4. Fry the croquettes until golden brown on all sides, about 5-7 minutes per side.
5. Remove from the pan and drain on paper towels. Serve hot.