01 - Preheat oven to 350°F (175°C). In a large bowl, combine ground beef, onion, garlic, breadcrumbs, milk, eggs, tomato paste, Worcestershire sauce, salt, and pepper. Mix gently until just combined.
02 - Transfer mixture to a parchment-lined loaf pan, pressing gently to form an even shape. In a small bowl, blend ketchup, brown sugar, and Dijon mustard. Spread topping uniformly over meat mixture.
03 - Bake in the preheated oven for 55 to 65 minutes until internal temperature reaches 160°F (71°C). Rest for 10 minutes before slicing.
04 - While loaf bakes, place potatoes in a large pot and cover with cold salted water. Bring to a boil over high heat, then reduce to a simmer and cook for 15–20 minutes, until fork-tender.
05 - Drain potatoes thoroughly. Return to pot and mash using a potato masher. Mix in warm milk, unsalted butter, salt, and pepper until smooth and creamy.
06 - Slice meatloaf and arrange with mashed potatoes. Serve hot.