Preheat your oven to 375°F (190°C).
In a skillet, melt butter over medium heat.
Add chopped onion and minced garlic, sauté until soft.
Stir in diced chicken and mixed vegetables.
Sprinkle flour over the mixture and stir well.
Pour in chicken broth and stir until thickened.
Mix in heavy cream and season with salt and pepper.
Transfer the filling into a greased casserole dish.
Cover with the pre-made pie crust and seal the edges.
Bake for 30-35 minutes until the crust is golden brown.