In a large skillet set over medium-high heat, add the ground beef and cook for about 4 minutes until browned, breaking it apart with a spatula. Add the diced onion and minced garlic, cooking for an additional 2–3 minutes until the onions are translucent.

Stir in the frozen mixed vegetables, tomato paste, Worcestershire sauce, and beef broth into the skillet. Season with salt and pepper, reduce heat to medium-low and let simmer for about 5 minutes.

In a large pot, bring salted water to a boil. Add diced potatoes and cook for 12–15 minutes, or until fork-tender. Drain and mash with butter, warm milk, salt, and pepper until creamy.

In a 9x13-inch baking dish, spread the savory beef filling evenly across the bottom. Dollop the creamy mashed potatoes on top and smooth them out.

Preheat oven to 400°F (200°C) and bake the assembled shepherd's pie for 20–25 minutes, or until the filling is bubbling and the potatoes turn golden brown.

Once baked, remove from oven and let sit for 5–10 minutes to set. Serve warm.