Heat oven to 350 degrees. Heat oil in a flame-proof casserole over medium-high heat. Add chicken pieces and cook, stirring, 5 minutes. Stir in onion, celery and carrots. Cook 5 minutes.Advertisement
In a small bowl or measuring cup, whisk broth, flour, salt and pepper. Pour into dish and bring to a simmer. Simmer 1 to 2 minutes, until thickened. Stir in dill and peas; cook 1 more minute.
Meanwhile, microwave potatoes for 10 minutes, as per package directions. Carefully transfer to a large bowl and add milk, butter, dill and salt. Mash until smooth. Dollop onto filling and gently spread to dish edges.
Transfer to oven and bake at 350 degrees for 15 minutes, or until bubbly. Cool slightly before serving.