Preheat oven to 400F. Line a medium sheet pan with parchment paper and arrange butternut squash in a single layer. Roast squash at 400F 15-20 minutes until just barely tender. Keep oven on.
In a medium skillet, heat olive oil over medium-high heat. Add onion and saute several minutes until tender. Add garlic, thyme, sage, and flour and cook an additional 1-2 minutes.
Reduce heat to medium and gradually begin to drizzle in chicken broth, whisking constantly until smooth. Add milk and heavy cream to skillet and bring to a low simmer over medium heat, stirring occasionally until sauce is thickened.
Remove sauce from heat and stir in turkey, peas, and roasted butternut squash. Season with salt and pepper to taste and pour into the bottom of an 8x8 baking dish. Set aside for a moment.
Remove pastry from fridge and turn onto a floured surface. Roll pastry out into a 8x8 square and cut into 9 pieces.
Place dough squares on a large parchment paper-lined sheet pan and brush with egg white-water mixture. Bake squares at 400F 15-20 minutes until a very light golden brown.
Carefully transfer dough squares to baking dish and arrange on top of filling. Bake pot pie at 400F an additional 25-30 minutes until filling is bubbly and pastry is golden brown.
Allow pot pie to cool several minutes then serve warm. Enjoy!