Peel and chop the potatoes into even chunks. Place them in a large pot, cover with salted water, and bring to a boil. Cook until tender and easily pierced with a fork, about 20 minutes.
While the potatoes cook, heat oil in a skillet over medium heat. Add chopped onion and minced garlic, sauté until fragrant and translucent, about 5 minutes, filling your kitchen with a warm aroma.
Add ground beef to the skillet, breaking it apart with a spatula. Cook until browned and cooked through, about 8 minutes. Stir in Worcestershire sauce and frozen peas, cook for another 2 minutes until peas are heated and incorporated.
Drain the cooked potatoes and return them to the hot pot. Add butter and warm milk, then mash until silky and smooth. Season with salt and pepper to taste, adjusting for flavor and creaminess.
Spread the savory beef mixture evenly in an ovenproof dish, smoothing the top with a spatula. Dollop the mashed potatoes over the filling and gently spread to cover completely.
Use a fork to create spooky ghostly patterns by swirling and drawing shapes into the mashed topping. This adds a fun, Halloween touch to your dish.
Place the dish in a preheated oven at 200°C (390°F) and bake for 20-25 minutes, until bubbling around the edges and the top is golden brown.
Remove from the oven and let sit for a few minutes to settle. Serve hot, marveling at the spooky patterns and hearty, comforting flavors.