In a large skillet over medium heat, add the ground beef. Cook until browned, breaking it apart with a wooden spoon, about 5-7 minutes.

Add the chopped onion and minced garlic to the skillet. Sauté until the onion is translucent, about 3-4 minutes.

Season the mixture with salt, black pepper, and oregano. Stir to combine.

Pour in the beef broth and add the frozen mixed vegetables if using. Bring to a simmer and cook for an additional 5 minutes, allowing the flavors to meld.

Remove from heat and set aside.

In a large pot, bring salted water to a boil. Add the cubed potatoes and cook until tender, about 15-20 minutes.

Drain the potatoes and return them to the pot. Add warm milk, melted butter, salt, and black pepper.

Mash the potatoes using a potato masher until smooth and creamy. Stir in the shredded cheddar cheese until melted and well combined.

Preheat your oven to 375°F (190°C).

In a greased 9x13 inch baking dish, spread the beef mixture evenly across the bottom.

Carefully spoon the mashed potatoes over the beef layer, spreading them out evenly with a spatula.

Sprinkle the remaining shredded cheddar cheese on top of the mashed potatoes.

Place the casserole in the preheated oven and bake for 25-30 minutes, or until the cheese is bubbly and golden.

Remove from the oven and let it cool for a few minutes before garnishing with fresh parsley.

Serve warm and enjoy your hearty ground beef and potato casserole!