Brown the ground beef, sausage, and onion in a large skillet or the insert of your slow cooker. Break up the meat as it cooks and add the garlic in the final minute or so, stirring until fragrant but not brown.
If cooking in a separate skillet, transfer the mixture to your slow cooker.
Add the remaining ingredients, except the fresh basil if using, and mix well.
Cover and cook on low for 6-8 hours or on high for 3-4 hours. Stir in the optional fresh basil at the end of the cooking time.
Serve over hot spaghetti noodles, or cool and then refrigerate or freeze for later use.