In a large bowl, whisk together the flour and water until smooth.
Add the grated cabbage, sliced chicken breast, sliced mushrooms, and green onion to the bowl. Mix well to combine.
Heat vegetable oil in a large non-stick skillet over medium heat. Pour the batter into the skillet, spreading it evenly into a round shape.
Cook for 3-4 minutes, or until the bottom is golden brown. Flip the pancake and cook for an additional 3-4 minutes.
In a small bowl, mix together the soy sauce, mayonnaise, ketchup, and Worcestershire sauce. Set aside.
Transfer the cooked okonomiyaki to a serving plate. Drizzle the sauce mixture over the top and sprinkle with bonito flakes and aonori.
Serve with pickled ginger on the side. Enjoy!