Preheat oven to 425°. Roll out puff pastry to 1/4-inch thick, and cut into 4 (6-inch) rounds to fit over 1-cup ovenproof bowls. Keep pastry covered and chilled.
Heat butter and oil in a medium saucepan over moderate heat; add flour, and cook, whisking constantly, for 1 minute. Add the milk in a slow stream while whisking constantly; bring the mixture to a simmer. Simmer for about 5 minutes or until thickened. Stir in next 6 ingredients (through pepper). Spoon mixture into ovenproof bowls. Top bowls with pastry, pressing against the outside edge of the bowls to seal. Place a sage leaf on top of pastry, and brush with egg white.
Bake pot pies on a baking sheet in middle of oven for 17 minutes or until pastry is golden brown. Serve hot.