Use a pizza cutter to slice the puff pastry into 8 equal-sized strips. Weave the strips into a square lattice and place on a greased baking sheet.
Bake in preheated oven until it just starts to brown, 10 to 15 minutes. Remove from oven and set aside.
Place the frozen peas, carrots, and corn in a microwave-safe bowl and cook in the microwave until cooked but still firm, 5 to 10 minutes.
Melt the butter in a skillet over medium heat. Stir in the flour and cook for 1 minute. Pour in the milk and stir while cooking until the mixture is smooth. Add the thyme and continue to cook and stir until the mixture thickens. Stir in the cooked vegetables and the chicken. Season with salt and pepper. Cook until heated through, stirring frequently, 5 to 7 minutes.
Pour the chicken mixture into a 2-quart baking dish. Lay the pastry lattice on top. Bake in the preheated oven until the crust is golden brown, 10 to 15 minutes.