Cook star anise and cinnamon in a large skillet over medium until fragrant, about 2 minutes. Transfer to a 6-quart slow cooker. Cut 1 onion in half; place in skillet, cut side down; add ginger. Thinly slice remaining onion, and set aside. Cook onion and ginger over medium, undisturbed, until onion is charred on cut side, 5 to 7 minutes. Transfer to slow cooker. Increase heat under skillet to medium-high. Add short ribs; cook until browned on all sides, 14 to 16 minutes. Remove skillet from heat; discard drippings. Pour 1 cup water into skillet, scraping to loosen browned bits. Pour through a strainer into slow cooker; discard solids. Stir in beef stock and salt. Cover and cook on low 8 hours.
Remove short ribs from slow cooker. Pour broth through a strainer lined with cheesecloth into a bowl; discard solids. Skim and discard fat from surface of broth; pour broth back into slow cooker. Remove meat from bones of short ribs; shred meat, and return to slow cooker. Stir in fish sauce and hoisin sauce. Turn slow cooker to high; cover to keep hot.
Cook noodles according to package directions. Place 1/2 cup cooked noodles in each of 4 bowls. Top each with 1 ounce raw sirloin and 1 1/4 cups hot broth (broth will cook meat). Top each with 1/3 cup short rib meat and 1/4 cup thinly sliced onion. Sprinkle with bean sprouts, basil, and mint. Serve with lime wedges; garnish with Fresno chile.