Preheat the oven to 375°F (190°C). This will ensure the oven is hot and ready for baking the pie.
Prepare the filling: In a large skillet, melt 3 tbsp butter over medium heat. Add 1 small chopped onion and 2 cloves minced garlic, sautéing for 2-3 minutes until fragrant.
Add chicken and vegetables: Stir in 2 cups of cooked chicken and 1 ½ cups of frozen mixed vegetables (peas, carrots, etc.), and cook for an additional 5 minutes.
Create the sauce: Pour in 1 cup of chicken broth and ½ cup of heavy cream. Stir and simmer for 5-7 minutes until the sauce thickens slightly.
Season the mixture: Add 1 tsp dried thyme, salt, and pepper to taste. Stir well to combine.
Assemble the pie: Unroll 2 Pillsbury pie crusts and place one in a 9-inch pie dish. Spoon the chicken mixture into the pie crust, then place the second crust on top, sealing the edges and cutting small slits on the top for ventilation.
Bake the pie: Place the pie in the preheated oven and bake for 30-35 minutes or until the crust is golden brown and the filling is bubbling.
Rest the pie: Let the pie cool for about 10 minutes before slicing and serving.
Garnish (optional): Sprinkle 1 tbsp fresh parsley on top of the pie for a fresh finish.