In a large bowl, combine the ground beef, panko breadcrumbs, sautéed onion and garlic, egg, Worcestershire sauce, dried thyme, dried parsley, salt, and pepper. Use your hands to gently mix everything together until just combined. Be careful not to overmix.

Using a spoon or ice cream scoop, portion out the meat mixture and roll it into 1-inch meatballs. Place the meatballs on a baking sheet lined with parchment paper.

Heat a tablespoon of olive oil in a large skillet over medium-high heat. Add the meatballs to the skillet in a single layer and sear on all sides until browned. You don't need to cook them all the way through at this point, just get a nice crust on them. Work in batches if necessary to avoid overcrowding the skillet.

Remove the meatballs from the skillet and set aside. Add the remaining olive oil to the skillet and sauté the remaining onion and garlic until softened. Stir in the flour and cook for 1 minute, stirring constantly.

Gradually whisk in the beef broth until smooth. Add the Worcestershire sauce, Dijon mustard, dried thyme, and bay leaf. Bring to a simmer and cook for 10-15 minutes, or until the gravy has thickened.

Return the meatballs to the skillet and nestle them into the gravy. Reduce the heat to low, cover, and simmer for 15-20 minutes, or until the meatballs are cooked through and the gravy has thickened further.

Remove the bay leaf and stir in the butter. Season with salt and pepper to taste.

Serve the Salisbury Steak Meatballs over mashed potatoes, rice, or egg noodles.