Cook the bacon in a 12-inch skillet over medium-high heat until crisp. Remove the bacon from the skillet. Pour off all but 1 tablespoon drippings.
Add the onion and cook in the hot drippings until tender, stirring occasionally. Add the garlic and beef and cook until the beef is well browned, stirring often to separate meat. Pour off any fat.
Stir the bacon, beef mixture, sauce and milk in a 6-quart slow cooker.
Cover and cook on HIGH for 4 to 5 hours. Serve with the spaghetti and cheese.
Make-Ahead: You may also cook this recipe on LOW for 7 to 8 hours.
Recipe Note: Reserve some of the cooking water from the spaghetti. You can use it to adjust the consistency of the finished sauce, if you like.