Place spare ribs, onion, ginger, garlic, coriander, cloves, and star anise in bottom of slow cooker. Pour chicken broth on top. In a bowl, mix together 3 cups water, the fish sauce, and brown sugar; pour into slow cooker. Cover and cook on high for 6 hours or low for 8 hours.
Remove spare ribs. Shred meat, discarding bones, tendons, and fat. Cover and set aside. Carefully pour remaining slow-cooker contents through a strainer over a pot with a lid; skim. Stir in salt. Cover and bring to a boil. Add rice noodles and cook 4 to 5 minutes, until softened.
Ladle some of the broth and noodles into each bowl. then top with shredded meat and, if desired, lime wedges, cilantro, scallions, bean sprouts, chili peppers, and Sriracha sauce.