Preheat the oven to 400°F. Heat a large skillet over medium heat and cook the ground beef until browned, breaking it up as it cooks.
Add the diced onion, carrots, and minced garlic to the skillet. Cook for 5 minutes until the vegetables soften.
Stir in the tomato paste, beef broth, salt, and pepper. Simmer for 10 minutes until the mixture thickens. Stir in the peas during the last 2 minutes.
While the filling simmers, boil the potatoes in a large pot of salted water for 15 minutes until tender. Drain and return them to the pot.
Mash the potatoes with the milk and butter until smooth. Season with salt and pepper.
Spoon the beef filling into a baking dish. Spoon the mashed potatoes into a piping bag or zip-top bag with the corner cut off.
Pipe the potatoes into ghost shapes over the filling. Press two peas into each ghost to form eyes.
Bake for 20 minutes until the potato ghosts turn lightly golden on the edges.