Preheat oven to 375F. Place milk in a large saucepan, set over medium heat, and whisk in flour until dissolved. Continue whisking over heat until thick and bubbling. Whisk in Worcestershire sauce, mustard, thyme, onion powder, salt and pepper. Cook, whisking constantly, 1 minute. Stir in turkey and vegetables; pour into a 13-by-9-inch baking dish. Cover top of casserole with 2 sheets of puff pastry placed end-to-end and overlapped slightly in the middle; trim off any excess that hangs over edges. If desired, cut excess dough into patterns or designs with cookie cutters and lay on top of pastry. Bake about 50 minutes or until top is browned and puffed and filling is bubbling. Let stand at room temperature 5 minutes before serving. Serves 6.