Preheat your oven to 400°F (200°C).
In a large mixing bowl, combine ground beef, finely minced onion, egg, breadcrumbs, Italian seasoning, chopped parsley, minced garlic, salt, and black pepper. Mix gently.
Shape the mixture into meatballs about 1 inch in diameter and place them on a baking sheet lined with parchment paper.
Bake for about 20-25 minutes until browned and cooked through, reaching an internal temperature of 160°F (70°C).
In a skillet over medium heat, add olive oil and sauté chopped onion until translucent. Stir in tomato paste, Italian seasoning, and minced garlic; cook for another minute.
Add diced tomatoes and tomato sauce to the skillet. Optionally add apple vinegar or broth/water and brown sugar. Let simmer on low heat.
Once meatballs are done baking, transfer them into the skillet with the sauce and gently stir to coat. Allow to simmer for about 10 minutes.