How to Make It
In a large bowl, whisk the flour into the eggs until a smooth batter forms. Add the shredded cabbage, sliced shiitake, 3/4 teaspoon of salt and 1 tablespoon each of the pickled ginger and bonito flakes. Mix until thoroughly combined. In a small bowl, combine the pork and shrimp and season lightly with salt and pepper.
In a small saucepan, whisk together the mirin, soy sauce, Worcestershire sauce and ketchup. Bring the sauce just to a boil, then simmer over moderately low heat until thickened slightly, about 5 minutes. Remove the saucepan from the heat.
In a 12-inch nonstick skillet, heat the vegetable oil. Stir the pancake mixture. Spoon four 1-cup portions of the pancake mixture into the skillet and flatten each into a 5-inch pancake about 1/2 inch thick. Cook the pancakes over moderate heat until they are golden brown on the bottom, 5 to 7 minutes. If the pancakes brown too quickly, lower the heat slightly.
Make a slight indentation in the top of each pancake and top each of them with one-fourth of the pork and shrimp, pressing down slightly. Carefully turn the pancakes over; if necessary, use two spatulas to hold the pancakes together. Cook the pancakes until they are golden brown on the second side and the pork and shrimp are cooked through, about 6 minutes.
Transfer the pancakes to a platter, pork and shrimp side up. Brush the pancakes with some of the sauce and top each of them with a little pickled ginger and some bonito and nori flakes. Serve the pancakes hot, with the remaining sauce and mayonnaise on the side.