Method
Heat the oven to 375°F (190°C) and lightly butter a 9x13-inch baking dish.
Melt the butter in a large skillet over medium heat. Cook the onion and carrots 6–8 minutes, until soft.
Stir in the turkey, gravy, thyme and pepper. Cook 3–4 minutes until warm and bubbling at the edges, then stir in the peas. Season with salt if needed.
Spread the filling in the baking dish and crumble the stuffing evenly over the top.
Microwave the mashed potatoes 2–3 minutes, then stir in warm milk a splash at a time until spreadable.
Spread the potatoes over the stuffing to the edges, make ridges with a fork and sprinkle with cheese if using.
Bake 30–35 minutes, until bubbling at the edges and 165°F (74°C) in the center. Broil 2–3 minutes for a browner top, watching closely.
Rest 10 minutes before serving.