Place potato chunks into a large pot and cover with cold water. Add a pinch of salt, bring to a boil, then simmer for 12–15 minutes until fork-tender. Drain well.

Return potatoes to the pot off the heat. Add butter and warm milk, then mash until smooth and creamy. Season with salt and pepper to taste. Keep warm while you make the gravy.

In a large skillet over medium-high heat, warm 1 tablespoon butter or oil. Add ground beef and cook, breaking it into pieces with a spatula, until browned and no longer pink (about 6-8 minutes). Spoon beef into a bowl and set aside.

Reduce heat to medium. Add remaining butter. Sauté chopped onion until translucent, about 4-5 minutes. Add garlic and cook for another 30 seconds until fragrant.

Sprinkle flour over the onions and garlic, stir to coat and cook for 1-2 minutes to remove the raw flour taste.

Slowly whisk in beef broth, scraping any brown bits from the pan. Add Worcestershire sauce and Dijon if using. Bring to a simmer and let thicken for 3–5 minutes. Add more broth if the gravy is too thick.

Return the cooked ground beef to the pan and simmer for 2–3 minutes to heat through and let flavors marry. Adjust seasoning.

Spoon mashed potatoes onto each plate and ladle the beef and gravy over the top. Garnish with chopped parsley.