Cook the chopped potatoes in salted water until fork-tender, about 15 minutes, then drain well.
Mash hot potatoes with butter, milk, garlic powder, salt, and pepper until smooth and creamy, adding a splash more milk if too thick for piping. Let cool slightly.
In a skillet over medium heat, sauté diced onion and garlic in olive oil for 2–3 minutes. Add ground beef and cook until browned.
Stir in tomato paste, Worcestershire sauce, frozen veggies, beef broth, salt, and pepper. Simmer 5–7 minutes until thickened.
Spoon the beef mixture into a 9×9 baking dish and smooth the top. Pipe cooled mashed potatoes over the filling in tall swirls like little ghosts, adding two black peppercorns for eyes.
Bake at 375°F for 20–25 minutes until the ghosts are lightly golden and the filling is bubbling. Cool slightly before serving.