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.

In a large bowl, whisk together 1¼ cups flour and baking soda. Stir in sweet potatoes, buttermilk, 1/2 teaspoon salt and 1/4 teaspoon pepper to make a thick batter. Do not over mix. Drop batter in large spoonfuls over chicken mixture and spread to cover almost to the edge of the pie dish. Bake until topping is golden brown and cooked through; about 30 minutes.