Drain the tofu and cut into three long, thin rectangular pieces.
Transfer tofu slabs to a clean kitchen towel and fold it over them. Place a baking sheet on top and weight it down, with a heavy item like a cast iron pan, to press out extra moisture. Press the tofu for 10-15 min.
Make Katsu Sauce: While the tofu presses, stir all ingredients for the katsu sauce together, until combined. In one shallow bowl, combine the ground flax and water, whisk to combine and slightly thicken. In another bowl, stir together flour, garlic powder, onion powder and season with salt and pepper. In a third bowl, add the Panko breadcrumbs. Set your breading station up like an assembly line, first the flour, then the flax mixture and finally the Panko. You may also want a baking sheet with a wire rack on it to transfer the coated tofu to.
Working with one tofu slab at a time, toss both sides in flour mixture, then dip into flax mixture. Lastly, toss in panko, turning and pressing to coat all sides. Transfer breaded tofu to a wire rack on a baking sheet. Repeat with remaining tofu.
In a large pan, cast iron works well, heat 2 tbsp oil over medium heat. When the oil is hot, add 1-2 slabs breaded tofu. Avoid overcrowding the pan, you may have to cook the slabs in two batches and add more oil as needed. Cook tofu for 3-4 min per side, until golden-brown and crisp all over. Transfer tofu to a wire rack. Repeat with remaining tofu cutlets. Tip: You can keep tofu warm on the wire rack, in a low temperature oven, until ready to serve. Once the slabs are all cooked, slice them into into ½-inch slices. Serve rice in bowls, then arrange cabbage, cucumbers and tofu on top. Drizzle katsu sauce over top, then sprinkle with sesame seeds and green onions.