In a large saucepan over medium heat, melt the butter. Stir in the flour, garlic powder, onion powder, salt, and pepper to create a roux. Cook for 1-2 minutes until golden.
Slowly whisk in the chicken broth and milk, stirring continuously until the mixture thickens and bubbles.
Add the shredded chicken and frozen mixed vegetables to the thickened sauce. Stir well to combine and let it simmer for about 5 minutes. Adjust seasoning with salt, pepper, and thyme.
Transfer the chicken and vegetable mixture to a 9-inch pie dish or a similar oven-safe dish.
Open the can of biscuit dough and separate the biscuits. Arrange them on top of the chicken mixture, covering the surface evenly.
Bake in the preheated oven for 20-25 minutes, or until the biscuits are golden brown and cooked through.
Remove from the oven and let it cool slightly before serving. Garnish with fresh herbs if desired.