Boil the potatoes in salted water until fork-tender (about 15 minutes).
Drain and mash with butter, milk, salt, and pepper. Set aside.
Heat a skillet over medium heat. Add the ground beef and cook until browned, breaking it apart with a spoon (5–7 minutes). Drain excess fat if needed.
Add the diced onion and garlic, cooking for 3–4 minutes until softened.
Stir in tomato paste, Worcestershire sauce, beef broth, and frozen vegetables. Let simmer for 5 minutes until slightly thickened.
Preheat your oven to 375°F (190°C).
Spread the beef mixture evenly in a baking dish.
Spoon the mashed potatoes on top, spreading them evenly with a spoon or spatula.
Bake uncovered for 20 minutes, or until the top is slightly golden.
Let it cool for a few minutes before serving.