Heat the oil in a large skillet over medium-high heat. Add the onion and cook for 1-2 minutes until fragrant, add the turkey, garlic and red pepper flakes. Cook and crumble until meat is brown and no longer pink. Drain any fat and transfer contents to a large 6-quart slow cooker.
Add the celery, carrots, diced tomatoes, tomato sauce, beef broth, Italian seasoning, fennel seeds, and bay leaf. Cover and cook on LOW for 3-4 hours or HIGH for 2 hours. Remove cover and stir in beans, zucchini, and pasta and cook on HIGH for an additional 30 minutes or until pasta is tender. Season with salt and black pepper to taste and discard bay leaf. Stir in balsamic vinegar if desired. If soup is too thick for your liking, add more water as needed
**NOTE about pasta – **If you plan to freeze this or serve the next day, leave out the pasta and cook on the side and then combine when you are ready to enjoy. This helps the pasta not get soggy.