Preheat the oven to 410°F. Lightly oil a baking sheet or line with parchment paper.

In a large bowl, combine 1 ½ pounds lean ground beef, 1 medium onion (minced), 1 large egg, 4 tablespoons breadcrumbs, 2 teaspoons Italian seasoning, 1 tablespoon chopped parsley, 2 cloves garlic (minced), ½ teaspoon salt, black pepper, and 1 teaspoon balsamic vinegar (if using). Mix well until evenly incorporated and mixture sticks together, but do not overmix or meatballs will be tough/dense.

Using about 2-3 teaspoons of mixture per meatball, roll into balls and place on the prepared baking sheet.

Bake for 10-12 minutes, or until lightly browned. Meatballs do not need to be cooked through at this point.

Heat 1 tablespoon olive oil in a large, oven-safe skillet over medium heat. Add 1 medium onion (chopped) and cook for 5 minutes until softened.

Stir in 2 tablespoons tomato paste, 2 teaspoons Italian seasoning, and 2 cloves garlic (minced). Cook for 2 minutes until fragrant.

Pour in ⅓ cup red wine (or broth/water), scraping up any browned bits.

Stir in 1 (14-oz) can diced tomatoes, 1 (14-oz) can tomato sauce, and ½ teaspoon brown sugar (if using). Let simmer on medium-low heat for 5 minutes, stirring occasionally. Season with salt + black pepper to taste.

Transfer the baked meatballs into the skillet with the sauce, making sure they are evenly coated.

Cover with lid for a thinner sauce or leave uncovered for a thicker sauce. Bake at 375°F for 10-15 minutes, or until the meatballs are fully cooked and the sauce is bubbling.

Sprinkle with fresh parsley and grated Parmesan before serving. Serve over spaghetti, mashed potatoes, or with crusty bread.