Pat the 3 pounds beef chunks dry and season with the 1 teaspoon salt and ½ teaspoon black pepper.
Heat the 2 tablespoons vegetable oil in a large skillet over high heat and sear the beef on all sides until deeply browned, working in batches. Do not skip this, that browning builds the deep savory flavor the whole dish is built on. Transfer the beef to the slow cooker.
Add the sliced onion, 6 cloves minced garlic, and 2 tablespoons grated ginger to the slow cooker with the beef.
In a bowl, whisk together the ½ cup soy sauce, ⅓ cup brown sugar, ¼ cup rice vinegar, 2 tablespoons hoisin, 1 tablespoon sesame oil, and 2 cups beef broth. Pour it over the beef, then tuck in the 1 star anise pod and add the ¼ teaspoon red pepper flakes.
Cover and cook on low for 8 hours, or on high for 4 to 5 hours, until the beef is fork tender and falling apart.
Fish out the star anise. Stir the 3 tablespoons cornstarch into the 3 tablespoons water until smooth, then stir it into the slow cooker. Cover and cook on high for another 15 to 20 minutes, until the sauce thickens into a glossy glaze that coats the beef. This is what turns thin braising liquid into the saucy finish you want.
Serve the beef and plenty of sauce over the 3 cups warm rice, topped with the sliced green onions and 1 tablespoon sesame seeds.