Peel and cut potatoes into chunks. Cover with cold salted water in a large pot, bring to boil over medium-high heat. Cook for about 15-20 minutes until fork-tender, drain, and mash with butter and milk until smooth.

Heat oil in a large skillet over medium heat. Cook diced onion until translucent, about 5 minutes. Add ground meat, browning for about 8-10 minutes. Stir in carrots, peas, Worcestershire sauce, salt, and pepper; cook until vegetables are tender, about 5 minutes.

Preheat oven to 400°F (200°C). Spread meat filling in a baking dish, then spoon mashed potatoes on top, spreading evenly. Create a texture pattern with a fork.

Bake in the preheated oven for about 25-30 minutes until the top is golden brown and bubbling. Allow to cool slightly before serving.