Place the peeled and chopped potatoes into a large pot, cover with salted water, and bring to a boil. Cook for about 15 minutes until tender, then drain and set aside.
Meanwhile, heat a skillet over medium-high heat. Add the ground meat and cook, breaking it apart with a spatula, until browned and caramelized, about 8-10 minutes. The meat should develop a deep, flavorful crust.
Add the diced onion and mixed vegetables to the skillet. Cook until the onion is golden and the vegetables are heated through, about 5-7 minutes. Stir in fresh thyme, salt, and pepper to taste.
Pour in the stock and simmer the mixture for 10 minutes, allowing it to thicken slightly and develop rich flavors. The filling should be bubbling and fragrant.
While the filling simmers, mash the drained potatoes with butter and cream until smooth or slightly lumpy, depending on your preference. Season with salt and pepper as needed.
Spread the meat and vegetable filling evenly into a baking dish. Top with the mashed potatoes, spreading gently with a spatula to cover completely. Sprinkle grated cheddar cheese over the top if using.
Bake in a preheated oven at 200°C (390°F) for 25-30 minutes, until the top is golden brown and bubbling around the edges. For an extra crispy crust, broil for 2-3 minutes at the end, watching carefully.
Remove from the oven and let rest for about 5 minutes. Slice into hearty portions, ensuring some of the crispy topping and bubbling filling make each serving special. Serve hot and enjoy the comforting aroma and rustic look.
