Prepare Panko topping: Heat 3 tablespoons of oil in a frying pan over medium heat, and cook Panko until nicely browned. Set aside.
Chop onion coarsely and set aside.
Make mashed potatoes: Peel and cut potatoes into small pieces. Put in a bowl, wrap with plastic wrap, and microwave for 5-6 minutes until soft. Mash with a potato masher, and season with salt and pepper.
Heat 1 teaspoon of oil in an iron skillet over medium heat, and cook ground chicken for a couple of minutes. Then add chopped onion and cook until tender, and season with salt and pepper. Add mashed potatoes and mix well. Add milk and mix. If you like the potato mixture to be softer, add extra milk 1 tablespoon at a time. Turn off the heat.
Flatten the surface of the potato mixture, and top with the Panko topping. Drizzle Tonkatsu sauce on top.