In butter saute onion and mushrooms until tender, about 5 minutes. Meanwhile boil carrots until tender, add peas about the last 2 min, drain. Add Flour to the onion and mushrooms and cook about 2 mins. Add broth and milk, cook until thicken and bubbly. Add Chicken, carrots and peas. Pour into baking dish. Cover with 1 puff pastry sheet.
Bake at 450 for 15 mins or until golden.