Preheat the oven to 375°F (190°C). Butter a 9x13-inch casserole dish and set aside.

In a large skillet over medium-high heat, melt 1 tablespoon butter. Add the chopped onion and sauté until translucent, about 3 minutes.

Add the ground beef and cook until browned, breaking it up with a spoon. Sprinkle in salt, pepper, and smoked paprika.

When the meat releases its juices and begins to caramelize, add minced garlic and cook for 30 seconds until fragrant.

Sprinkle flour over the meat and stir to combine.

Pour in the milk while stirring, scraping any browned bits from the pan.

Stir in sour cream and Dijon mustard until smooth. Reduce heat and let the sauce thicken slightly.

Arrange half of the sliced potatoes in the bottom of the prepared dish.

Spoon half of the beef mixture over the potatoes, scatter half of the frozen peas and carrots (if using), and sprinkle 1 cup of shredded cheese.

Repeat with the remaining potatoes and beef, then top with the remaining cheese.

Tightly cover the dish with foil and bake for 35 minutes.

Remove foil and bake another 10 minutes until the top is golden and bubbly.

Let the casserole rest for 5 minutes before serving, garnish with chopped parsley or chives.