How to Make It
Heat oven to 425° F. Place the potatoes in a large pot and add cold water to cover.
Bring to a boil and add 1 teaspoon salt. Reduce heat and simmer until just tender, about 15 minutes. Drain the potatoes and return them to the pot.
Add the milk, butter, and ¼ teaspoon each salt and pepper to the potatoes and mash.
Meanwhile, heat the oil in a large skillet over medium-high heat. Add the beef and cook, breaking it up with a spoon, until browned, 5 to 6 minutes.
Stir in the ketchup, Worcestershire sauce, ½ teaspoon salt, and ¼ teaspoon pepper. Add the peas and carrots.
Transfer the beef mixture to 4 individual baking dishes and top with the mashed potatoes. Bake until golden brown, 15 to 20 minutes.