Homemade Chicken Pot Pie with Flaky Crust

Preheat the oven to 400°F (200°C). Lightly grease a 9-inch deep-dish pie plate or square baking dish.
Season chicken with salt and pepper and cook until done (or use pre-cooked chicken). Dice or shred and set aside.
In a large skillet over medium heat, melt butter. Add chopped onion and cook 4–5 minutes until translucent. Stir in garlic and cook 30 seconds until fragrant.
Sprinkle flour over the onions and cook, stirring constantly, for about 2 minutes to form a blond roux.
Gradually whisk in chicken broth, then heavy cream. Bring to a gentle simmer, whisking, until thick enough to coat the back of a spoon, 4–6 minutes.
Stir in cooked chicken, diced carrots, and frozen peas. Add salt, black pepper, and thyme (if using). Simmer 1–2 minutes. Remove from heat and let the filling cool 10 minutes.
Pour the filling into the prepared dish. Roll out the pie crust and lay it over the filling, tucking and crimping edges. Cut 4–5 small slits to vent. Brush lightly with milk or cream if desired.
Bake at 375°F (reduce oven temperature after filling is added) for 30–35 minutes, until the crust is deep golden and the filling bubbles through the vents. Tent edges with foil if browning too quickly.
Cool 10–15 minutes before slicing and serving to allow the filling to set for clean slices.