In a large pot, melt butter over medium heat. Sauté diced onion until translucent.
Stir in the flour and cook for 1-2 minutes to form a roux.
Gradually whisk in chicken broth, ensuring no lumps remain. Bring to a gentle simmer.
Add shredded chicken and frozen vegetables to the pot. Simmer for 10-12 minutes until vegetables are tender.
Pour in milk and heavy cream, stirring well. Season with salt and pepper to taste.
Serve hot, topped with crumbled pie crust for a deliciously comforting touch.