In a large cast iron Dutch oven, heat the oil over medium-high heat. Slowly whisk in the flour and stir constantly until you have a light brown roux (it should be the color of peanut butter).
Add the onions, bell peppers, and garlic and cook, stirring frequently, until the vegetables are wilted. Add the chicken and cook until well browned on all sides, 5 to 10 minutes. Add the stock and blend well. (You may need to add more stock or water to achieve a stew-like consistency.) Add the butter beans and season to taste with salt and pepper. Bring to a rolling boil, reduce the heat to a simmer, and cook until the chicken is cooked through and tender, 30 to 45 minutes.
Stir in the scallions; taste and adjust the seasoning if necessary.