Gather all ingredients.
Preheat oven to 400 degrees F (200 degrees C) with rack in bottom third position. Heat bacon in a medium cast-iron skillet over medium and cook, stirring occasionally, until crisp, about 10 minutes. Using a slotted spoon, transfer bacon to a paper towel-lined plate. Reserve drippings in a small bowl.
While bacon cooks, stir together rice, salt, and water in an 8-cup microwave-safe container. Microwave on HIGH until al dente, about 4 minutes. Drain to eliminate excess water, if needed, and transfer to a large bowl.
Thinly slice scallions, keeping dark and light green parts separate. Return 1 tablespoon of bacon drippings to the skillet over medium heat and add light green parts of scallions, cabbage, garlic, and ginger; cook, stirring often, until softened and fragrant, 3 to 4 minutes. Remove from heat and transfer to bowl with rice.
Add half (about 1/4 cup) of the reserved bacon, 1/4 cup mayonnaise, and 2 tablespoons soy sauce to bowl with rice and cabbage; toss until fully combined.
Heat oil and remaining 2 tablespoons bacon drippings in a skillet over medium heat until just sizzling. Add rice mixture to the hot skillet and use a spoon to flatten and cover the bottom of the skillet. Use end of a wooden spoon handle to gently poke 5 to 6 holes in the rice, going all the way to bottom of skillet. This will allow steam to escape and a crust to form on the bottom.
Continue cooking over medium heat until rice sizzles and begins to brown on edges, about 4 minutes. Transfer to preheated oven and cook on the bottom third rack until bottom of rice is nice and crisp, 30 to 35 minutes.
While rice bakes, whisk together ketchup, Worcestershire sauce, and remaining 1/2 tablespoons soy sauce in a small bowl; set Okonomiyaki sauce aside until ready to use.
Remove skillet from oven. Drizzle evenly with some of the ketchup mixture and remaining 1/4 cup kewpie mayonnaise. Sprinkle with sesame seeds, dark green parts of scallions, reserved bacon, aonori, and bonito flakes, if using. Serve immediately.
