01 - Set oven temperature to 400°F (200°C).
02 - In a large skillet over medium heat, melt butter. Add diced onion and sauté until softened, about 3–4 minutes.
03 - Sprinkle all-purpose flour over onions and stir for 1 minute to form a roux.
04 - Gradually whisk in chicken broth and whole milk. Add salt, ground black pepper, and dried thyme. Cook, stirring frequently, until the sauce thickens, approximately 3–4 minutes.
05 - Incorporate cooked chicken, peas and carrots mix, and corn kernels. Simmer mixture for 2 minutes.
06 - Spread chicken and vegetable mixture evenly in a 9x13-inch baking dish.
07 - Place biscuit dough pieces evenly atop the filling.
08 - Bake in preheated oven for 20–25 minutes, or until biscuits are golden brown and cooked through.
09 - Allow casserole to cool for 5 minutes before serving.