Family Secret Meatballs and Sauce

Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.

Mix together the ground beef, rolled oats, 6 tablespoons of sweet onion, milk, salt, and black pepper until thoroughly combined. Roll the mixture into 2-inch meatballs, and place meatballs into the prepared baking dish.

In a bowl, mix the ketchup, Worcestershire sauce, sugar, 4 1/2 tablespoons of sweet onion, apple cider vinegar, and water until the sugar has dissolved, and pour the sauce evenly over the meatballs.

Bake the meatballs and sauce in the preheated oven until the sauce is bubbling and thickened and the meatballs are no longer pink inside, about 1 1/2 hours. An instant-read thermometer inserted into the center of a meatball should read at least 160 degrees F (70 degrees C).