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. Pre-heat the oven at 375°F. 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. Place a portion of spaghetti on each plate, top with sauce, meatballs, and grated parmesan cheese. Serves 6 to 10