Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
MAKE THE ROASTED VEGGIES:
In a medium bowl, toss the squash, onion, carrot and potatoes with the olive oil and a generous sprinkle of salt and pepper. Spread on the baking sheet and roast for 20 to 30 minutes, just until tender. Remove from the oven and let cool. Increase the oven temperature to 425 degrees F.
ASSEMBLE THE POT PIES:
On a work surface, roll out the Puff Pastry to ¼-inch-thick, and cut into 4 (6-inch) rounds or squares, depending on the shape of your 1 cup ovenproof dishes. Use the dish to turn upside down on the pastry and cut out the circles a little larger than the dish. Keep the pastry covered and refrigerated until ready to use.
In a medium saucepan, melt the butter and oil over moderate heat; add the 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 the lemon juice, chicken, roasted veggies, basil, salt and pepper. Spoon the mixture into small ovenproof dishes. Top the bowls with pastry, pressing against the outside edge of the dishes to seal. Place a sage leaf on top of each pastry and brush with egg white.
Bake the pot pies on a baking sheet on the middle rack in the oven for 15 to 17 minutes, or until the pastry is golden brown. Serve hot.
Notes
*Change up the roasted vegetables as you wish- trying things that are suited to your taste.