In a large mixing bowl, combine the ground beef, bread crumbs, chopped onion, egg, salt, and pepper. Mix until just combined—don’t overwork the meat. Shape the mixture into 1½-inch meatballs and place them on a plate or tray. Heat a large nonstick skillet over medium heat. Add the meatballs in a single layer and cook, turning occasionally, until browned on all sides and fully cooked through, about 10–12 minutes. Drain excess grease if needed. In a medium bowl, whisk together brown sugar, water, ketchup, apple cider vinegar, soy sauce, and cornstarch until smooth and well combined. Pour the sauce over the cooked meatballs in the skillet. Reduce the heat to low and simmer, stirring gently, until the sauce thickens and coats the meatballs—about 3–5 minutes. Serve the sweet and sour meatballs hot over rice or noodles, or enjoy them as a party appetizer. Garnish with chopped green onions or sesame seeds, if desired.