Preheat the oven to 400°F (200°C). In a mixing bowl, combine ground beef, breadcrumbs, egg, onion, garlic, Worcestershire sauce, salt, black pepper, and thyme. Mix gently.

Shape mixture into 1-inch meatballs and place on a lined baking sheet. Bake for 20 minutes until browned and cooked through.

While baking, heat olive oil in a skillet over medium heat. Sauté onion and garlic until translucent, about 3–4 minutes.

Add beef broth and Worcestershire sauce to the pan. Bring to a simmer. Prepare a slurry with cornstarch and water; stir into gravy until thickened. Season with salt and pepper.

In a pot, boil salted water and add chopped potatoes and garlic. Cook until fork-tender, about 15–18 minutes. Drain and return to pot.

Add milk, butter, salt, and parsley to potatoes. Mash until creamy. Adjust seasoning as needed.

Combine baked meatballs with gravy and simmer on low for a couple of minutes.

Serve mashed potatoes topped with Salisbury steak meatballs and drizzle with extra gravy.