Preheat the oven to 375°F.

Put diced potatoes in a large pot. Cover with cold water, at least 2 inches above the potatoes. Bring to a boil, cover, and cook about 15 minutes or until fork tender.

Drain potatoes and leave them in the pot, uncovered, for 10 minutes to let steam escape. This prevents watery mash.

Add 8 tablespoons salted butter, ½ cup whole milk, ½ cup grated parmesan, 1 teaspoon kosher salt, and 1 teaspoon black pepper to the potatoes. Mash until smooth and creamy.

While potatoes cook, heat a large skillet over medium. Melt 2 tablespoons unsalted butter.

Add 1 pound ground beef and brown, breaking it into small crumbles until no pink remains.

Add the diced sweet onion and cook about 10 minutes, until softened and translucent.

Stir in 3 minced garlic cloves, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon kosher salt, and 1 teaspoon black pepper. Cook 30 seconds to bloom the spices.

Sprinkle 2 tablespoons all-purpose flour over the mixture and stir, cooking 1 minute to remove raw flour taste.

Pour in 1 cup beef stock and 2 teaspoons Worcestershire sauce. Stir to combine and bring to a gentle simmer to thicken.

Add 12 ounces mixed frozen veggies and stir until evenly distributed. Remove from heat.

Spread the meat-and-veg filling in an even layer in a 9x13 baking dish.

Spoon mashed potatoes over the filling and smooth the top with a spatula or fork.

Bake for 40–45 minutes until the top is dark golden brown and the edges are bubbly.

Let cool for 15 minutes before serving. Garnish with chopped parsley if desired.