Preheat oven to 375 F degrees. Heat a BIG oven safe skillet over MED heat. Add oil and ground beef and cook about 4 minutes, or until cooked through, breaking it up into small crumbles as it cooks. Once browned, transfer to a plate. If needed, drain excess grease before adding the beef to a plate.
Add butter to same skillet. Once melted, add onion, carrots, mushrooms, garlic, dried oregano, and 1/2 the salt. Cook, stirring occasionally, about 8-10 minutes.
Add in tomato paste, stirring to combine, and cook 5-7 minutes, until tomato paste is a dark, brick red color (this really brings out the tomato flavor).
Return cooked ground beef to the skillet, then stir in beef stock, Worcestershire sauce remaining salt, and black pepper. Simmer for a few minutes, until mixture is slightly thickened.
Top with mashed potatoes (see recipe notes section), and Parmesan cheese. Use a fork to rough up the top of the potatoes, then dot with the butter pieces.
Bake 40-45 minutes, until bubbly and potatoes are browned. Let cool 5-10 minutes before serving. Sprinkle with minced fresh parsley and enjoy!