In bowl, combine ginger, garlic, soy sauce, sugar, and red pepper flakes. Heat skillet over medium heat, add oil and coat. Add 1/2 of the beef and cook until browned through; about 3 minutes. Remove and cook other 1/2 of the beef. Add onions, beef, and soy sauce mixture and cook for 1 minute. Stir in spinach and cook for 30 seconds. Top with sesame seeds before serving.