In a large bowl, whisk together egg and 1 cup dashi or water. Add flour, baking powder, salt, and black pepper and whisk until combined. Fold in cabbage and green onions with a large spoon.
In a large skillet over medium heat, heat 2 tablespoons of oil. Scoop half of batter into skillet and spread into an even layer, about ¼-inch thick. Cook until golden on the bottom, about 3 minutes.
Lay half of pork belly evenly over the top. Flip and lower heat to medium-low. Do not press down on the pancake as it cooks. Add more oil if the pan looks dry. Cook until browned, about 3 minutes. Transfer to a large plate, then repeat with remaining batter.
Make sauce: In a small bowl, whisk together ketchup, Worcestershire, honey, and salt.
Drizzle okonomiyaki with sauce and mayo. Sprinkle with green onions, nori flakes, bonito flakes, and picked ginger. Serve while hot.
