Combine first 5 ingredients in a small bowl. Heat wok or skillet over high heat. Add oil to pan; swirl to coat. Add half of the beef to pan, and cook for 3 minutes or until browned. Remove the beef from pan and repeat procedure with remaining beef. Add onions; cook for 1 minute. Add soy sauce mixture; cook 1 minute. Return beef to pan; cook 1 minute. Stir in spinach, and cook 30 seconds. Top with sesame seeds.