Chop the onion. Heat 1 tablespoon of oil in a frying pan on a medium heat and sauté the onion until translucent. Add the pork and sauté for about 4 minutes, or until the pork is cooked through. Season with the soy sauce and mirin. Cook the potatoes in their skins, ideally whole, in a large saucepan of boiling water. (If you are short on time, cut them in half.) Drain and peel. Mash the potatoes roughly in a bowl with a fork or spatula, without turning them into a purée. Add the pork and onion mixture and season with salt and pepper. Shape into 8 croquettes.
Take 3 shallow bowls: put the flour in the first one, beaten eggs in the second, and panko in the third. Dip the patties in the flour, then in the egg and finally in the panko. Pour a 3 cm (1¼ in) depth of oil into a pan and heat to 180°C (350°F). Fry the croquettes until nicely browned. Drain on paper towels and serve immediately, with or without the tonkatsu sauce.