Cut or shred the cabbage into thin slices. Wash under cold running water and drain completely. Keep it cool in the fridge.
To make miso sauce, heat 1 Tbsp mirin and 2 Tbsp sake in a small saucepan over high heat and let the alcohol evaporate.
Lower the heat and add 2 Tbsp sugar, ½ cup (120 ml) dashi, and ¼ cup (72 g) Hatcho Miso.
Combine well with a silicone spatula. Cook for 3 minutes on low heat, stir frequently to avoid burning on the bottom of the saucepan. When the sauce is thickened, turn off the heat and set aside.
To make Tonkatsu, remove extra fat from pork loin and make a couple of slits on the connective tissue between the meat and fat. The reason why you do this is that red meat and fat have different elasticity, and when they are cooked they will shrink and expand at different rates. This will allow the meat to stay nice and flat when deep frying and prevent from curling up.
Pound the meat with a meat pounder, or if you don’t have one then just use the back of the knife to pound. Mold the extended meat back into original shape with your hands.
Season both sides of the meat with salt and pepper.
In a large bowl, whisk together 1 large egg and 1 Tbsp water.
Dredge in flour and remove excess flour.
Then dip in egg mixture and dredge in panko. After removing excess panko, press gently. While deep frying panko will “pop up” so at this moment they don’t have to be fluffy.
Heat oil in a pot over medium-high heat and wait till oil gets 350ºF (180ºC). If you don’t have a thermometer, stick a chopstick in the oil and see if tiny bubbles start to appear around the tip of the chopstick. Alternatively, you can drop one piece of panko into the oil, and if it sinks down to the middle of the oil and comes right up, then that’s around 350ºF (180ºC) as well. When the oil reaches that temperature, gently submerge the pork loin into the oil. Keep watching the oil’s temperature and make sure it doesn’t go over 350ºF (180ºC) or the pork katsu will look burnt.
Deep fry for 1 minute on one side and flip to cook the other side for 1 minute. If your meat is thinner than ¾ inch, then reduce to 45 seconds for each side.
Now take out the meat and remove the excess oil by holding it in a vertical position for a few seconds. Place on top of the wire rack (if wire rack is not available, use paper towel) and let it sit for 4 minutes. The hot oil on the exterior is slowly cooking the meat as it sits. Please do not cut to check whether the inside is cooked or not. We need to keep the panko shell on to retain the heat. While waiting, you can scoop up fried crumbs in the oil with a fine-mesh strainer.
After resting for 4 minutes, bring the oil back to 350ºF (180ºC) of oil again and deep fry the meat for 1 minute (about 30 seconds each side).
Remove from the oil and poke the meat with a chopstick and if clear liquid comes out then it’s done. Drain the oil by holding the meat vertically again for a few seconds. Then leave it on top of the rack or paper towel for 2 minutes. If you have to use the paper towel, try to keep the meat in a vertical position so it does not get soggy on one side.
Cut Tonkatsu into large pieces by pressing the knife directly down instead of moving back and forth. This way the breading will not come off. Transfer to a plate and serve the cabbage on the side. You can pour the sauce before serving or serve the sauce on the side. Enjoy immediately.
You can keep the sauce in an airtight container in the refrigerator for up to 2 weeks and in freezer. You can also use this sauce for marinating the fish or use as a dipping sauce for hot pot, konnyaku, and steamed vegetables.