In a medium bowl, mix together the ground beef, egg, shredded onion and breadcrumbs. Roll ground beef mixture into 1-1 ½ inch balls. Set aside.
In another bowl, mix together the crushed tomatoes, garlic, basil and parmesan cheese.
Spread ½ cup of the tomato sauce mixture in the bottom of your slow cooker. Place the meatballs on top, and then cover with remaining sauce.
Cook on low for 8 hours.
Serve over pasta or on long rolls.