When butter is melted, add garlic cloves and cook for 1 minute.
Whisk in flour, thyme, sage, salt, and pepper to form a paste.
Add chicken broth to the paste, whisking until it is smooth. Heat through.
Pour broth over ribs in slow cooker.
Cover, and cook on high for 4-5 hours, until meat is fork-tender.
To make a gravy, remove all ribs from slow cooker and keep warm. Pour broth from slow cooker into a gravy separator. Pour broth into the saucepan with as little grease as possible.
Whisk in up to 1/4 cup all purpose flour to thicken the broth. (The exact amount of flour will depend on the amount of broth and desired thickness.) Add flour slowly, and whisk continuously to avoid lumps.
Simmer gravy over low heat until it reaches desired thickness. Serve over ribs.