Place the peeled and chopped potatoes in a large pot of salted water. Bring to a boil over high heat and cook until tender, about 15 minutes. Drain well and set aside.
While the potatoes cook, heat the large skillet over medium-high heat. Add a splash of oil, then toss in diced onions and minced garlic. Sauté until fragrant and translucent, about 2 minutes.
Add the ground beef to the skillet. Break it apart with a spatula and cook until browned and cooked through, about 8-10 minutes. Season with salt and pepper, then stir in Worcestershire sauce for extra depth.
Stir in diced carrots and thawed frozen peas into the meat mixture. Cook for another 3-4 minutes, until vegetables are slightly softened and flavors meld. Remove from heat.
Return drained potatoes to the hot pot. Mash with a potato masher until smooth and fluffy. Mix in butter, warm milk, and a pinch of salt and pepper until creamy and well combined.
Spread the meat and vegetable filling evenly in a baking dish. Top with the mashed potatoes, smoothing the surface with a spatula. If desired, sprinkle grated cheddar cheese on top for a golden crust.
Bake in a preheated oven at 200°C (400°F) for 20-25 minutes, until the top is golden and slightly crispy, and the filling is bubbling around the edges.
Remove from oven and let rest for about 5 minutes. This helps the filling settle and makes slicing easier. Serve hot, garnished with fresh herbs if desired.
