1. In a skillet over medium heat, brown the ground beef until fully cooked. Drain any excess fat.
2. Add the diced onion and minced garlic to the skillet and sauté until the onion is translucent.
3. Transfer the beef mixture to the slow cooker.
4. Add the diced bell pepper, sliced carrots, and sliced celery to the slow cooker.
5. Stir in the diced tomatoes, tomato paste, beef broth, oregano, basil, salt, and pepper.
6. Cover and cook on low for 6-7 hours or on high for 3-4 hours.
7. About 30 minutes before serving, add the broken spaghetti noodles to the slow cooker and stir well.
8. Cover and cook until the noodles are tender, about 25-30 minutes.
9. Serve hot, topped with grated Parmesan cheese.

Variations & Tips
For a vegetarian version, omit the ground beef and add more vegetables such as zucchini or mushrooms. If you have picky eaters, you can blend the sauce to hide the vegetables before adding the spaghetti. For a spicier kick, add a pinch of red pepper flakes. You can also substitute ground turkey or chicken for the beef for a lighter option.