Preheat oven to 400 degrees.
Rinse and scrub potatoes to remove any visible dirt and then peel using a vegetable peeler. Cut them into roughly 1-inch chunks.
Add the potatoes to a large Dutch oven or pot and then cover the potatoes with at least 1-inch of water. Sprinkle in a pinch or two of salt and bring the water to a boil. Once boiling, reduce the heat to low and keep a rapid simmer for 10 minutes, or until the potatoes are fork tender.
Once cooked, drain the potatoes in a colander in the sink and then add them back to the pot along with the milk, butter, Parmesan cheese, ¾-1 teaspoon of salt, to taste, ¼ teaspoon of black pepper, and ¼ teaspoon of garlic powder. Using a potato masher, mash all of this together until it's smooth and creamy with very little chunks remaining. Set this aside until you’re ready to use it.
Add oil to a large skillet or saucepan along with the diced onion and celery. Saute over medium for 3-4 minutes and then add in 3 cloves of finely minced garlic. Continue sauteeing for an additional 30 seconds or until the garlic becomes fragrant.
Push the vegetables to the side of the skillet and add the ground beef. Cook for 7-8 minutes over medium heat, or until there is no pink remaining. Use a potato masher to tenderize the meat and break it apart into finer crumbles.
Add in the tomato paste, flour, Worcestershire sauce, herbs, salt, pepper. Mix this all together until well combined.
Pour in the beef broth, reduce the heat to low, and let the mixture simmer for about 10 minutes or until it has thickened up and is no longer runny.
During the last 2 minutes of cooking, stir in the previously frozen and now thawed mixed vegetables.
Spray a 7 x 10-inch baking dish with non-stick cooking spray and then add in the ground meat filling, spreading it out to cover the bottom of the dish. Then place dollops of the mashed potatoes on top, evenly spaced. Use a spatula to spread this out to completely cover the ground meat layer. Use the side of the spatula to make little peaks in the potatoes if you’d like some of the edges to crisp up in the oven.
Bake the shepherd’s pie in the preheated oven for 25-30 minutes, or until the edges begin to brown slightly. You can also pop it under a broiler for a few minutes at the end to get an extra crispy topping.
Let it rest at room temperature for 10-20 minutes in order for the ground meat filling to thicken back up.
Serve it with a sprinkle of finely chopped fresh parsley and enjoy!