In a large mixing bowl, combine the ground beef, egg, breadcrumbs, Parmesan cheese, garlic powder, Italian seasoning, salt, pepper, and parsley. Mix gently with your hands or a wooden spoon until just combined. Do not overmix.
Scoop about 2 tablespoons of the meat mixture and roll it into a ball. Place one ½-inch cube of mozzarella in the center and shape the meat around the cheese to completely enclose it.
Repeat with the remaining meat mixture and mozzarella.
In a 6-quart slow cooker, combine the crushed tomatoes, onion powder, garlic, Italian seasoning, sugar, salt, pepper, and beef broth. Stir until combined.
Gently place the stuffed meatballs into the sauce in a single layer.
Cover and cook on high for 2–3 hours or on low for 4–6 hours.
Serve warm.
Notes
Refrigerate the leftovers in an airtight container for up to 5 days.