Preheat oven to 400°F. Line a baking sheet with parchment paper or lightly grease it.

In a large mixing bowl, combine breadcrumbs and milk. Let sit for 2 minutes to create a panade that will keep meatballs tender.

Add ground beef, chopped onion, minced garlic, egg, Worcestershire sauce, salt, pepper, dried parsley, onion powder, and garlic powder. Mix gently with hands just until combined, being careful not to overwork the meat.

Shape mixture into meatballs roughly 1.5 inches in diameter, keeping them uniform in size. Place on prepared baking sheet.

Bake for 20 minutes or until meatballs are cooked through and reach 160°F internally.

While meatballs bake, place peeled and chopped potatoes in a large saucepan. Cover with water by 1 inch, add salt, and bring to a boil. Cook for 15 to 20 minutes until fork-tender.

For the gravy, place a frying pan over medium heat. Add flour and toast for 1 minute, stirring constantly, until fragrant but not browned.

Gradually whisk in beef broth, adding it slowly and stirring constantly to prevent lumps from forming.

Stir in soy sauce and Dijon mustard. Season with salt and pepper to taste. Simmer for about 5 minutes, stirring occasionally, until gravy thickens enough to coat the back of a spoon.

Once meatballs are cooked, add them to the gravy and spoon sauce over them to coat well. Let simmer together for 1 to 2 minutes.

Drain cooked potatoes and return to pot. Add minced garlic, butter, and milk. Mash until smooth and creamy. Stir in chopped fresh parsley and season with salt and pepper to taste.

Serve Salisbury steak meatballs over garlic herb mashed potatoes with generous spoonfuls of gravy. Garnish with additional fresh parsley if desired.