Preheat your oven to 400°F (200°C). In a large mixing bowl, combine ground beef, breadcrumbs, milk, chopped onion, minced garlic, egg, Worcestershire sauce, salt, pepper, dried parsley, onion powder, and garlic powder. Mix gently with your hands until just combined, about 30 seconds.

Shape the mixture into meatballs about 1.5 inches in diameter, making approximately 16-18 meatballs. Place them on a baking sheet with about an inch between each one. Bake for 20 minutes until they reach an internal temperature of 160°F and develop a golden-brown crust.

While the meatballs bake, place chopped potatoes and whole garlic cloves in a large saucepan and cover with cold salted water. Bring to a boil, then reduce heat and simmer for 15-20 minutes until fork-tender.

Drain the potatoes and garlic thoroughly, then return them to the hot saucepan. Add butter and warmed milk. Mash until smooth and creamy. Stir in chopped parsley, salt, and pepper. Keep warm.

For the gravy, heat a dry frying pan over medium heat. Add flour and cook, stirring constantly, for about 1 minute until light golden. Gradually whisk in beef broth to prevent lumps. Add soy sauce and Dijon mustard, then simmer for 5 minutes until thickened.

Add the baked meatballs to the gravy, gently turning them to coat completely. Let them simmer together for 2-3 minutes. Serve the meatballs over garlic herb mashed potatoes.