Soak the dried mushrooms for the sauce in boiling hot water for 15-20 minutes until completely rehydrated. Do not discard the water.Slice the mushrooms and set aside. See video below on how to make tofu katsu. Drain water from the tofu by using a tofu press or wrapping it in a paper towel and then placing a heavy flat surface (a board or plate) on top. Leave for 15 minutes until water has been absorbed by the towel.
Slice the tofu into ½ -inch thick square slabs. You may be able to slice one tofu into 3 slabs, depending on the thickness of your tofu. Season with ¼ tsp salt.
Prepare the batter by mixing all the ingredients until smooth. In another plate or tray, add the breadcrumbs.
Place each tofu in the batter then coat into the breadcrumbs. Repeat this step for the rest.
Heat the oil in a frying pan. I used a cast iron pan. Once hot (you can test this by putting a little bread crumbs and it will sizzle when hot), add in the tofu.
Fry for around 6-8 minutes, flipping halfway, over medium heat until golden brown on both sides. Take the tofu out from the oil then turn off the heat.
Let cool for 10-15 minutes then slice diagonally into ¾ -inch thick strips.
Carefully remove the oil from the pan. Add in the mushroom water/broth, onions, and sliced mushrooms, Leave to simmer for 2-3 minutes. Add in the soy sauce, mirin, sake, and sugar. Mix well and cook until the sugar has dissolved.
Add in the sliced tofu katsu. Using a spoon, carefully scoop the sauce and pour over the tofu until they are well coated. Leave to cook until the tofu has absorbed almost all the liquid. Turn off heat.
Place the tofu over some steamed Japanese rice. Feel free to garnish with some spring onions and sesame seeds, if desired. Enjoy while hot!