Prepare the mashed potatoes: Boil the quartered potatoes in salted water for 20 minutes or until tender. Drain and return them to the pot. Add the milk and butter, then mash until smooth and creamy. Season with salt and pepper, cover, and set aside to keep warm.

Cook the ground beef and gravy: In a skillet over medium heat, cook the ground beef until browned. Drain excess fat, then add the diced onion and minced garlic (and mushrooms if using). Cook until the onions soften.

Sprinkle the flour over the beef, stirring to coat. Cook for 2 minutes, then gradually stir in the beef broth. Add Worcestershire sauce and thyme, simmer for 5 minutes until the gravy thickens. Season with salt and pepper to taste.

Assemble the dish: Spoon a generous amount of mashed potatoes onto each plate. Top with the ground beef and gravy mixture.

Serve hot and enjoy!