Preheat oven to 375 degrees.
Stir soup, milk and mayonnaise together in a heavy saucepan over low heat until heated through.
Add turkey, vegetables, onion, salt, pepper, and herbs (if using), and combine well, cooking another 1-2 minutes.
Pour mixture into a greased 9-inch pie plate or rectangular pan of similar size.
Top with pastry, crimping along the edges to seal.
Cut slits to let steam escape.
Brush with melted butter.
Bake 20-30 minutes or until pastry is golden brown.
Let stand 10 minutes before serving.