1. In a large bowl, gently combine the ground beef, panko breadcrumbs, grated onion, minced garlic, garlic powder, kosher salt, black pepper, dried mustard, Worcestershire sauce, A1 sauce, and slightly beaten egg. Mix with your hands until everything is just combined—don’t overmix.

2. Roll the mixture into 16 to 18 evenly sized balls.

3. Heat the olive oil in a large skillet over medium heat. Add the meatballs in a single layer and cook for about 6 to 8 minutes, turning occasionally to brown all sides.

4. Remove the meatballs to a plate.

5. In the same skillet, add butter over medium heat and let it melt. Toss in the sliced onions and sauté for about 5 minutes until they soften. Then, add the mushrooms and cook for another 5 minutes or until they become nicely browned.

6. Stir in the minced garlic, dried thyme, black pepper, and dried mustard, allowing it to cook for about a minute until fragrant.

7. Pour in 2 cup of beef stock along with the Worcestershire and A1 sauces, stirring to combine. Bring the mixture to a boil.

8. In a small bowl, whisk the flour with the remaining 2 tablespoon of beef stock to create a smooth slurry. Gradually add this to the skillet, stirring continuously until the gravy thickens to your desired consistency.

9. 9. Return your meatballs to the skillet with the gravy, reduce the heat to low, and cover. Let them simmer for about 10 to 12 minutes.

10. In a large pot, add the diced Yukon gold potatoes and cover them with cold water. Sprinkle in kosher salt and bring to a boil. Once boiling, reduce the heat and simmer for 30 to 35 minutes until tender. Drain the potatoes and return them to the pot off the heat to dry.

11. In a separate bowl, combine the softened butter with minced garlic, grated Parmesan, chopped parsley, garlic powder, black pepper, and paprika. Mix until smooth and well incorporated.

12. Pass the hot, drained potatoes through a ricer into a large mixing bowl. Add the prepared garlic butter and 2 teaspoon of kosher salt, stirring until the butter melts into the warm potatoes. Gradually pour in the warmed half-and-half until you achieve that creamy texture.

13. To serve, spoon a generous mound of garlic herb mashed potatoes onto each plate. Ladle the Salisbury Steak Meatballs with mushroom gravy generously over the potatoes.