1. In a skillet over medium heat, brown the ground beef until fully cooked. Drain excess fat.
2. Add the chopped onion and minced garlic to the skillet with the beef. Cook until the onion is translucent.
3. Transfer the beef mixture to the slow cooker.
4. Stir in the tomato sauce, diced tomatoes, oregano, basil, salt, and pepper.
5. Cover and cook on low for 4-5 hours.
6. About 30 minutes before serving, stir in the uncooked pasta shells.
7. Cover and cook on high for 30 minutes, or until the pasta is tender.
8. Stir in the cheddar and mozzarella cheeses until melted and well combined.

9. Sprinkle the Parmesan cheese on top before serving.
Variations & Tips
For a bit of a kick, try adding a pinch of red pepper flakes or a dash of hot sauce. You can substitute ground turkey or chicken for the beef if you prefer a lighter option. For a veggie boost, consider adding chopped bell peppers or mushrooms when you add the onion and garlic. If you have leftover cooked pasta, you can stir it in during the last 15 minutes of cooking instead of using uncooked pasta shells.