In medium nonstick saucepan over medium heat, cook turkey 5 minutes or until no longer pink. Stir in soup and poultry seasoning.
In measuring cup, blend milk and cornstarch until smooth. Stir into turkey mixture; cook until mixture begins to boil. Stir in peas and onions; cook 2 to 3 minutes or until thoroughly heated.
Place 1 cup turkey mixture in each of 4 individual microwave-safe or ovenproof casseroles. Pipe or spoon 1/2 cup mashed potatoes over top of each serving.
Bake at 400°F. for 15 to 20 minutes or until thoroughly heated.