To the hot pan, add the carrots, celery, and onions. Cook for 6 minutes or until the vegetables are softened and the onions are translucent. Stir in the garlic and cook for 1-2 more minutes. Stir in the butter and sprinkle with the flour. Stir until there are no flour lumps and the vegetables are coated with the butter/flour mixture. Pour in the chicken stock while stirring quickly and scraping the bottom of the pan to release and cooked on flavor.