Stir 2 of the minced garlic cloves, 1 large egg, ½ cup panko breadcrumbs, ¼ cup whole milk, 1 tablespoon of the fresh thyme leaves, 1 tablespoon of the Worcestershire sauce, ¾ teaspoon kosher salt, and ¼ teaspoon of the black pepper together in a large bowl. Add 1 ½ pounds ground beef and use your fingers to combine the meat and flavorings. Handle the meat gently and do not overmix.

Form the mixture 14 to 16 (about 2-inch) meatballs (about 2 ½ tablespoons or 2 ounces each). Arrange on a large plate in a single layer.

Melt 1 tablespoon unsalted butter in a 12-inch cast-iron or stainless steel pan over medium heat. Add the meatballs and cook until browned on two sides, 6 to 8 minutes per side. Return to the plate (the meatballs may not be cooked through).

Add 1 diced medium yellow onion and the remaining 1 tablespoon fresh thyme leaves to the pan. Cook, scraping the bottom of the pot to loosen any browned bits, until the onions are softened, about 3 minutes. Add the remaining 2 minced garlic cloves and cook until fragrant, about 30 seconds. Sprinkle in ¼ cup all-purpose flour and stir until it coats the onions, leaves a film on the bottom of the pan, and begins to brown, about 1 minute.

Whisk in 1 (14.5-ounce) can beef broth, 1 tablespoon soy sauce, the remaining 1 tablespoon Worcestershire sauce, and the remaining ½ teaspoon black pepper until smooth. Return the meatballs and any liquid on the plate to the pan and bring to a boil. Reduce the heat to maintain a simmer and cook until the gravy thickens and an instant-read thermometer inserted into the center of a meatball registers at least 165°F, 8 to 10 minutes.

Turn off the heat and stir in 1 teaspoon balsamic vinegar. Garnish with more fresh thyme leaves if desired. Serve over mashed potatoes, polenta, cooked egg noodles, or rice.