Stir 2 of the minced garlic cloves, 1 large egg, ½ cup panko breadcrumbs, ¼ cup whole milk, 1 tablespoon of the Worcestershire sauce, 1 teaspoon of the kosher salt, and ¼ teaspoon of the black pepper together in a large bowl. Add 1 ½ pounds ground beef and mix until just combined; do not overmix.

Divide the mixture into 15 portions (about 2 ½ tablespoons or 2 ounces each). Roll each one between your hands to form a smooth ball. Arrange on a large plate in a single layer.

Melt 2 tablespoons of the unsalted butter in a 12-inch cast iron or nonstick pan over medium heat. Add the meatballs and cook until browned on two sides, 3 to 5 minutes per side. Return to the plate (the meatballs will not be cooked through).

Add 2 tablespoons of the unsalted butter to the pan. Once melted, add 8 ounces quartered mushrooms and cook, stirring frequently, until the mushrooms are deeply browned, 6 to 8 minutes.

Melt the remaining 1 tablespoon unsalted butter in the pan. Add 1 diced medium yellow onion, 1 teaspoon paprika, the remaining ¾ teaspoon kosher salt, and the remaining ¼ teaspoon black pepper. Cook, stirring occasionally, until the onions are softened, 4 to 6 minutes.

Stir in the remaining 2 minced garlic cloves and 2 tablespoons all-purpose flour, and cook until the garlic is fragrant, about 30 seconds. Add ½ cup dry white wine and scrape up any browned bits from the bottom to the pan. Cook until the wine is nearly evaporated, 20 to 30 seconds.

Stir in 1 (14.5-ounce) can low-sodium beef broth, 1 tablespoon Dijon mustard, and the remaining 2 teaspoons Worcestershire sauce. Bring to a simmer.

Stir in ½ cup sour cream until fully incorporated (don’t worry if it looks broken or separated at first). Return the meatballs and any accumulated juices on the plate to the pan and bring to a boil. Reduce the heat to maintain a simmer. Cook, stirring occasionally and flipping the meatballs halfway through, until the sauce thickens and an instant-read thermometer inserted into the center of a meatball registers at least 165°F, 10 to 12 minutes total.

Garnish with finely chopped parsley or chives if desired. Serve over cooked egg noodles, polenta, or mashed potatoes.