Mix together soy sauce, mirin, sake, sesame oil, ginger, and garlic in a dish wide enough to fit all the tofu.
Gently squeeze out any excess water from the tofu. Cut into slabs or cubes and place into the dish with the seasonings. Gently press down on the tofu so they soak up some of the marinade. Flip and repeat so both sides have soaked up the marinade.
Coat tofu in flour, submerge in the cornstarch/water mixture, then coat in breadcrumbs. I like to use one hand to place the tofu in the cornstarch/water and the other hand to coat with breadcrumbs. This way the breadcrumbs don't stick all over your hand.
Place into your air fryer, making sure none of the tofu slabs touch. Cook for 10-15 minutes, until browned and crispy. You will probably have to cook them in batches.
In the meantime, prepare onion sauce in a small bowl by mixing onions and ketchup.
Top katsu with onion sauce, or Japanese tonkatsu sauce if preferred, and serve immediately.