Preheat oven to 400°F. Heat oil in a large skillet over medium heat. Add onions, 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook, stirring occasionally, until golden, 6-8 minutes. Add vegetables and cook until softened and hot throughout, about 10-15 minutes. Sprinkle 1/4 cup flour over vegetables, stir well and cook 2 minutes. Whisk in broth, bring to a boil and then reduce heat to medium low and simmer until thickened, 2 to 3 minutes. Stir in chicken, 1 tablespoon salt, and 1/2 teaspoon pepper. Transfer filling to a deep pie dish or baking dish; set aside.