Chicken Pot Pie

1 lb. boneless, skinless chicken breasts
1/4 tsp. salt
1/2 of 1 crust from (1) 5-oz. package of refrigerated piecrust
1 egg white, lightly beaten with 1 tsp. water
2 medium carrots, peeled and thinly sliced
1 cup frozen corn kernels
1 cup frozen green peas
1 cup frozen small white onions
1/2 cup evaporated skim milk
3 Tbs. all-purpose flour
1/2 tsp. butter
1/4 tsp. ground black pepper

Bring chicken breasts, 1/8 tsp. salt and enough water to cover to a simmer in large saucepan over medium-high heat. Reduce heat to low and poach chicken until juices run clear, about 15 minutes. Cool chicken and cut into bite-size pieces. Reserve 1 cup poaching liquid.
Preheat oven to 425 degrees. Lightly coat baking sheet and four 1-cup oven-proof dishes with cooking spray.
Dust counter lightly with flour. Unfold piecrust on surface and cut in half. Roll out crust 1/8 inch thick. Cut out 4 squares to fit on top of pies. Brush tops of squares with egg white mixture. Transfer to baking sheet and bake until crisp and golden-brown, about 12 minutes. Transfer to wire rack to cool.
Meanwhile, cook carrots in boiling water until tender, about 5 minutes; drain. Rinse frozen vegetables with hot water; drain. Whisk evaporated milk and flour in small bowl until smooth. Melt butter in medium saucepan over medium heat. Whisk in milk mixture, then reserved poaching liquid.
Cook until sauce thickens and boils, about 5 minutes. Stir in chicken, carrots, corn, peas, onions, pepper, and remaining salt. Cook until heated through, about 3 minutes; divide among the 4 dishes and bake until filling is bubbling, about 15 minutes. Top each with a pastry square.