Preheat your oven to 200°C (400°F).
In a large pan, cook the ground beef over medium heat until browned.
Add the chopped onion and diced carrots, and sauté until softened.
Stir in the tomato paste, then pour in the beef stock and let the mixture simmer for 15 minutes.
Meanwhile, boil the peeled and cubed potatoes until tender. Drain and mash with butter and warmed milk. Season with salt and pepper.
Transfer the beef mixture to a baking dish, and top with the mashed potatoes, spreading evenly.
Bake in the preheated oven for 20-25 minutes or until the top is golden brown.
Let it rest for a few minutes before serving.