Preheat your oven to 400°F (200°C). In a large bowl, combine ground beef, breadcrumbs, egg, chopped onion, minced garlic, Worcestershire sauce, salt, black pepper, and dried thyme. Mix until just combined.

Shape the meat mixture into 1-inch meatballs and place them on a baking sheet lined with parchment paper. Bake for 20 minutes, or until browned and cooked through.

Meanwhile, in a large skillet, heat olive oil over medium heat. Add sliced onion and garlic, sautéing until the onion is translucent, about 5 minutes.

Stir in beef broth and Worcestershire sauce, bringing the mixture to a simmer. Slowly whisk in the cornstarch slurry and continue to simmer until the gravy thickens. Season with salt and pepper to taste.

For the mashed potatoes, place chopped potatoes and garlic in a large pot. Cover with cold water and bring to a boil. Cook until potatoes are tender, about 15 to 20 minutes.

Drain the potatoes and garlic, then return them to the pot. Add milk and butter, and mash until smooth. Stir in salt, black pepper, and fresh parsley.

Add the cooked meatballs to the skillet with the gravy, gently tossing to coat.

Serve the Salisbury Steak Meatballs over a generous scoop of garlic herb mashed potatoes, drizzling with extra gravy.