Place the potato chunks in a large pot and cover them with cold water by about 1 inch. Add a pinch of salt, then bring the pot to a boil. Cook the potatoes for 15 to 18 minutes, or until they are fork-tender.

Drain the potatoes well and return them to the pot. Mash them with the warm milk, butter, sour cream, and salt until smooth and creamy. Set aside while you make the filling.

Preheat the oven to 400°F (200°C). Melt the 2 tbsp butter in a large oven-safe skillet over medium heat. Add the onion and carrots, and cook for 6 to 7 minutes until softened. Stir in the garlic and cook for 1 minute more.

Sprinkle the flour over the vegetables and cook, stirring, for 1 minute. Slowly pour in the broth while stirring to combine. Simmer for 4 to 5 minutes, until the mixture thickens. Stir in the cooked turkey, peas, thyme, salt, and pepper, then remove the pan from the heat.

If your skillet is oven-safe, keep the filling in the pan. If not, transfer it to a 9-inch deep baking dish. Spoon the mashed potatoes evenly over the top, then spread to the edges to seal. Use a fork to make decorative ridges, adding cheddar cheese on top if desired.

Bake for 20 to 25 minutes, until the filling is bubbling and the potato topping is golden brown. Let it rest for 10 minutes before serving.