In a large skillet, cook the ground beef over medium heat until browned. Add onion, carrots, and garlic, cooking until the vegetables are tender. Stir in the tomato paste, broth, and Worcestershire sauce; simmer for 10 minutes. Season with salt and pepper.
If not using leftover mashed potatoes, boil peeled potatoes in salted water until soft. Drain and mash with butter. Season with salt to taste.
Preheat oven to 400°F (200°C). Spread beef mixture in a baking dish, top with mashed potatoes, creating peaks with a fork.
Bake for 25 minutes until potatoes are golden and filling is bubbling. Cool slightly before serving.
Notes
Store leftovers in a fridge-safe container for up to 3 days, or freeze for up to 3 months. Reheat at 350°F (175°C) before serving to keep it delightful.