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

In a large bowl, combine lean ground beef, minced onion, egg, breadcrumbs, Italian seasoning, chopped parsley, minced garlic, salt, black pepper, and balsamic vinegar. Mix well until evenly incorporated.

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

Stir in tomato paste, Italian seasoning, and minced garlic. Cook for 2 minutes until fragrant.

Pour in red wine, scraping up any browned bits.

Stir in diced tomatoes, tomato sauce, and brown sugar. Let simmer on medium-low heat for 5 minutes, stirring occasionally. Season with salt and 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.