In slow cooker, stir together broth, onion, fish sauce, cinnamon, star anise, ginger, coriander, sugar, 2 teaspons salt. Add steak, cover and cook on low heat setting for 8 hours or high heat for 4 hours. Steak should be very tender.
Transfer the steak to a cutting board and shred into bite sized chunks with fingers or two forks.
Line a sieve with cheesecloth (if desired) and strain broth from slow cooker into a bowl. Discard the solids, and return broth and shredded steak back into slow cooker. Cover and cook on low for 30 minutes to warm through.
While soup is reheating prepare the rice noodles. Boil water, turn off heat and then add noodles until noodles are just cooked. Drain and rinse in old water. Prepare toppings.
To serve place noodles in bottom of bowl and ladle soup over top. Top with bean sprouts, jalapeños, lime wedges, basil, cilantro and Sriracha sauce.