1. Combine first 5 ingredients in a small bowl.
2. Heat a wok over high heat. Add oil to pan and coat. Add beef to pan, and cook until browned. Remove from pan.
3. Add broccoli and cook until slightly softened. Add onions and cook for 1 minute. Add soy sauce mixture and cook 1 minute. Return beef to pan; cook 1 minute. Add spinach, and cook until wilted. Top with sesame seeds.