For the meatballs: Heat olive oil over medium heat in a medium sauté pan. Add onions, celery, carrots and garlic. Cook until vegetables are soft and garlic is aromatic, approximately 15 minutes; let cool.
Combine bread, egg and milk in a large bowl. Add onion mixture, ground turkey and turkey sausage; mix well. Season with salt and pepper.
Preheat the oven at 375 degrees. Form mixture into 2-1/2-inch balls. Place on a lightly oiled baking sheet, do not let touch. Bake for 20 minutes or until cooked through.
For the sauce: Put the spinach in the bottom of a 4-quart slow cooker. Pour both cans of tomatoes, tomato paste, basil, garbanzo beans and cooked meatballs into the slow cooker. Cover and cook all ingredients on low for 8-10 hours.
When ready to serve, boil a large pot of water. Add the spaghetti and cook according to package directions. Drain.