Scrub the potatoes clean, and peel them. Cube them into small, even chunks. Fill your stockpot with cold water to cover all of the potatoes and then about 2 inches of extra. Add 1/2 teaspoon of salt to the water.
Cover the potatoes and bring to a boil. Allow potatoes to boil for about 10-15 minutes or until fork tender. Once tender, drain any excess water over the sink. Return potatoes to the pot; add butter, milk, salt, and pepper. Mash using a hand mixer, or potato masher until light and fluffy; set aside.
In a large, deep-walled skillet, over medium-high heat, add 1 tablespoon of butter, and onion. Cook for about one minute stirring occasionally before adding in the frozen vegetables. Add in the meat, and turn the heat up to high. Use your wooden spoon to break up the meat as it cooks. It will go from bright pink to a brown coloring.
Add in Worchestshire sauce, broth, garlic powder, onion powder, salt, and pepper. Cover pan, and bring to a simmer over medium-low heat for around 15 minutes. This is not a thick gravy so there will still be moisture in the pan.
Transfer meat and vegetable mixture to a large 9″ x 13″ casserole dish.
Layer mashed potatoes on top of the meat and veggie mixture. Use a rubber spatula to smooth over the mixture.
Bake uncovered for 30 minutes. The pie will have a nice crust on top from the buttery, baked potatoes.
Allow to sit for 5 minutes before serving warm.