In a large skillet, over medium high heat cook onion, celery, and carrots until slightly soft, approximately 5 minutes. Combine cooked turkey with vegetables in casserole dish. Add frozen peas. Using same skillet, melt 2 tablespoons butter. Whisk in flour, salt, pepper, and poultry seasoning. Add chicken broth and stir until thickened. Pour into casserole dish and stir to combine. Spread mashed potatoes over meat mixture. Bake 30 minutes.