Place the beef chuck roast cubes into the slow cooker.
In a medium bowl, whisk together the soy sauce, brown sugar, rice vinegar, water, minced garlic, and grated ginger until the sugar dissolves.
Pour the teriyaki sauce mixture over the beef in the slow cooker, ensuring the beef is well coated.
Cover and cook on low for 6 hours, or until the beef is tender and easily shredded.
Once cooked, remove the beef from the slow cooker and transfer to a large bowl, shredding it with two forks.
In a small bowl, mix the cornstarch with cold water until smooth to create a slurry.
Pour the cooking liquid from the slow cooker into a saucepan and bring to a simmer over medium heat.
Slowly whisk in the cornstarch slurry and cook for 2-3 minutes until the sauce thickens to a glaze consistency.
Return the shredded beef to the saucepan with the thickened sauce and stir to coat evenly.
Drizzle sesame oil over the beef and stir to combine.
Serve the beef teriyaki garnished with sliced green onions and toasted sesame seeds.