Preheat oven to 350°F. Line a 10-inch loaf pan with parchment paper so that it hangs over on either side. Grease with cooking spray and set aside.
Set a large skillet over moderate heat with the butter. Once melted, add in the onion, carrot, and celery and cook until softened, about 5 minutes. Stir in the garlic and a large pinch of salt and pepper. Cook for another minute or so. Remove from the heat and allow to cool slightly.
In a large bowl, combine the ground sirloin, eggs, Worcestershire sauce, tomato paste, breadcrumbs, peas, tomato sauce, parsley and another large pinch of salt and pepper. Do not over-mix or you'll end up with a tough meatloaf. Transfer mixture to the prepared pan and spread out evenly. Bake for 25 minutes, or until browned and almost cooked through.
Meanwhile, place the cubed potatoes in a saucepan and fill with cold water. Set over moderate heat and bring to a boil. cook until fork tender, about 12 to 15 minutes. Drain and return to the hot pot. Add the butter and milk and mash with a potato masher until as smooth as possible. Season with garlic, a large pinch of salt, pepper and the cheese. Stir until combined.
Remove the meatloaf from the oven and top with the mash potatoes. Don't be afraid to really pack it in. Smooth out evenly and return to the oven to finish cooking, another 20 to 25 minutes. The internal temperature should be 160°F. Remove from the oven and allow to cool slightly in pan. Lift out with the parchment paper, slice and serve. Enjoy!