Place cubed potatoes and smashed garlic cloves in a large pot of salted cold water; bring to a boil and simmer for 15-20 minutes until fork-tender.
In a large mixing bowl, combine ground beef, breadcrumbs, egg, 2 tbsp milk, 1 tsp Worcestershire sauce, onion powder, garlic powder, 1 tsp salt, and 0.5 tsp black pepper.
Form the meat mixture into 16 uniform meatballs, approximately 1.5 inches in diameter.
Heat 1 tbsp butter in a large skillet over medium-high heat; sear meatballs until browned on all sides (approximately 6-8 minutes), then remove from pan and set aside.
In the same skillet, melt 2 tbsp butter and add sliced onions and mushrooms; sauté for 5-7 minutes until caramelized.
Whisk in 2 tbsp flour and cook for 1 minute to remove raw flour taste, then gradually whisk in beef broth, ketchup, mustard, and remaining 1 tsp Worcestershire sauce.
Return meatballs to the skillet, reduce heat to low, and simmer for 10 minutes until the gravy has thickened and meatballs reach an internal temperature of 160°F.
While meatballs simmer, drain potatoes and garlic; mash thoroughly while incorporating remaining 3 tbsp butter and 0.5 cup warm milk.
Fold fresh parsley and chives into the mashed potatoes and season with salt and pepper.
Serve 4 meatballs and a generous portion of gravy over the garlic herb mashed potatoes.