How to Make It
Whisk together flour, dashi, and eggs until well incorporated. Fold in cabbage and chopped bonito flakes.
Melt butter in a 10-inch nonstick skillet or flat griddle over medium. Spread batter evenly in skillet, and top evenly with pork belly slices in a single layer. Cover and cook until bottom is browned, 6 to 8 minutes.
Slide pancake onto a large plate or cutting board. Place skillet over pancake and, holding plate and skillet firmly together, invert pancake into skillet, pork side down. Cook over medium until pork is browned and pancake is heated through, about 10 minutes.
Invert okonomiyaki onto plate, pork side up. Brush or drizzle with okonomiyaki sauce, and dollop or drizzle with mayonnaise. Sprinkle with ground bonito flakes, beni shoga, and nori. Cut into wedges, and serve immediately.