To the skillet, add the butter over medium heat. Once melted and frothy, add the onion and season with salt. Cook, stirring often, until golden brown and softened, about 8-10 minutes. Next, turn the heat to high for 1-2 minutes or until the onions begin to turn dark brown around the edges. Finally, add the chicken stock and scrape up any browned bits stuck to the bottom. Bring to a boil and then reduce heat and add the beans.