Shred the cooked chicken into bite-sized pieces and combine with frozen mixed vegetables in a large mixing bowl.
Pour in the chicken broth and heavy cream, then add garlic powder, onion powder, dried thyme, salt, and pepper. Stir well until evenly coated.
Transfer the filling to a 9-inch pie dish and spread evenly across the bottom.
In a separate bowl, combine biscuit mix, milk, and melted butter. Mix gently until just combined, then fold in shredded cheddar cheese.
Spoon the biscuit mixture over the chicken filling in the pie dish and spread evenly. Sprinkle remaining shredded cheddar cheese on top.
Preheat oven to 375°F and bake for 25-30 minutes until biscuits are golden brown and filling is bubbling. Let cool for 5 minutes before serving.