Peel and chop the potatoes into evenly sized pieces, then place them in a large pot. Cover with cold salted water and bring to a boil. Once boiling, reduce to a gentle simmer and cook for about 15 minutes until fork-tender. Drain and set aside to cool slightly before mashing.

In a mixing bowl, combine the hot, drained potatoes with butter, milk, garlic powder, salt, and pepper. Blend until smooth and creamy. Allow to cool slightly for piping later.

In a large skillet, add olive oil over medium heat, then sauté the diced onion and minced garlic for about 2-3 minutes until translucent. Add the ground beef, breaking apart as it cooks until browned, about 5-7 minutes. Stir in tomato paste and Worcestershire sauce.

Incorporate mixed vegetables into the beef mixture with beef broth. Season with salt and pepper to taste, then simmer on low heat for about 5-7 minutes until the filling thickens.

Preheat oven to 375°F (190°C). In a greased baking dish, spoon the beef filling into the bottom and smooth the top. Fill a piping bag with mashed potatoes and pipe ghostly shapes on top.

Bake the assembled shepherd's pie for approximately 20-25 minutes, or until potato tops are lightly golden and filling is bubbling.