Preheat the broiler too high. Line a large baking sheet with aluminum foil and set aside.
In a large mixing bowl combine the ground beef, breadcrumbs, parsley, garlic, onion, egg, parmesan cheese, salt and pepper. Mix until just combined.
Shape beef mixture into 1 inch meatballs and place on the prepared baking sheet.
Broil the meatballs until just browned for about 8 minutes. They do NOT need to be cooked all the way through.
While the meatballs are cooking, add the marinara sauce, diced tomatoes and tomato sauce to the crock pot. Stir to combine.
Add meatballs to the slow cooker and gently stir to combine.
Cook on low for 6-8 hours or on high for 3-4 hours.
Serve meatballs on top of cooked spaghetti. Top with parmesan cheese and minced parsley if desired. You can also serve these alone as an appetizer.