Preheat the oven to 375°F (190°C).
In a large saucepan, melt the butter over medium heat.
Whisk in the flour, salt, black pepper, dried thyme, and garlic powder until smooth; cook for 1-2 minutes to form a roux.
Gradually whisk in the chicken broth and milk, stirring constantly until the mixture thickens and bubbles, about 5-7 minutes.
Remove the sauce from heat and stir in the cooked chicken and thawed mixed vegetables until evenly combined.
Pour the chicken and vegetable mixture into a 9×13-inch casserole dish and spread evenly.
Separate the biscuit dough into 8 biscuits and place them evenly on top of the chicken mixture.
Bake uncovered for 30-35 minutes or until the biscuits are golden brown and the filling is bubbly.
Remove from the oven and let cool for 5 minutes before serving.
To freeze: Assemble the casserole as directed but do not bake. Cover tightly with plastic wrap and aluminum foil. Freeze for up to 3 months.
To bake from frozen: Remove plastic wrap and keep foil on. Bake at 375°F (190°C) for 60-70 minutes or until heated through and biscuits are golden. Remove foil for the last 10-15 minutes to brown the biscuits.