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 lidded pot; 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 bowls. Top with shredded meat and, if desired, lime wedges, cilantro, scallions, bean sprouts, chile peppers and sriracha sauce.