Start by melting the butter in a large skillet over medium heat. Add in the chopped onions and sauté until they become soft and translucent, about 5 minutes. Toss in the minced garlic and cook for another minute. The aroma will make your cooking space feel welcoming.

Next, stir in the frozen carrot medallions and let them cook for about 3-4 minutes. Sprinkle in the marjoram, smoked paprika, salt, and pepper. This adds a fragrant touch to your mixture.

Crumble your leftover meatloaf into the pan. Stir it well to combine, allowing it to absorb the flavors. Once everything is mixed nicely, sprinkle the flour over the mixture and stir for another minute. This will help thicken the filling.

Now, pour in the beef stock, ketchup, mustard, and Worcestershire sauce. Bring this to a simmer and allow it to thicken. Keep stirring occasionally to prevent sticking at the bottom.

Once thickened, mix in the frozen peas and half of the chopped parsley. This adds a pop of color and freshness to your filling. Take it off the heat and prepare for the topping.

In a separate bowl, combine the pre-prepared mashed potatoes with sour cream and Parmesan cheese. Stir thoroughly until it's creamy and smooth.

Spread the meat mixture into a casserole dish evenly. Then dollop the mashed potato mixture on top. Use a spatula to smooth it out or make decorative peaks—whatever calls to your inner chef.

Preheat your oven to 400°F (200°C) and bake the casserole for about 25-30 minutes, until the top turns golden brown.

Allow it to cool for a few minutes before serving. Sprinkle the remaining parsley on top for that fresh finish.