In a large slow-cooker, add steak. Add beef broth, soy sauce, brown sugar, sesame oil, Sriracha, garlic, and green onions. Cover and cook on low until beef is tender and cooked through, 3 1/2 to 4 hours. When the steak is tender, spoon a few tablespoons of the slow-cooker broth into a bowl and whisk with cornstarch. Pour into slow cooker and toss with the beef until combined. Add broccoli and cook, covered, 20 minutes more. Garnish with sesame seeds and green onions and serve over rice.