Steps

1. Grate 1/4 onion.
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2. In a bowl, combine the grated onion and 1/3 tsp salt.
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3. Add 250g ground pork and beef mix and 1 tbsp potato starch to the bowl.
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4. Gently mix with a rubber spatula until the grated onion and potato starch are evenly distributed. (This is the key!) Mixing gently without kneading creates a meaty and satisfying texture.
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5. Roughly divide the hamburg steak mixture into 4 portions.
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6. Heat vegetable oil (to taste) in a frying pan. Place 1/4 of the mixture onto the pan and flatten the surface with a rubber spatula or the back of a ladle. Repeat for the remaining portions, arranging them in the pan.
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7. Smooth the surface of the hamburg steaks in the pan and shape the sides as desired.
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8. Cook the hamburg steaks until golden brown.
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9. While the hamburg steaks are cooking, trim the tough ends off 1/2 pack of shimeji mushrooms and separate them lengthwise.
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10. Once one side of the hamburg steaks is golden brown, carefully flip them using the side of the pan. (This is the key!) The potato starch locks in the juices, and by not finely chopping the onion, fewer gaps are created, preventing juices from escaping.
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11. Cook the other side until golden brown.
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12. Once the hamburg steaks are cooked through, turn off the heat and transfer them to a serving plate.
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13. Utilize the flavorful drippings left in the pan from cooking the hamburg steaks. Lightly stir-fry the separated shimeji mushrooms. If the pan is too dirty, wipe it gently before stir-frying.
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14. Once the mushrooms are tender, add 2 tbsp cooking sake (rice wine), 1 tbsp mirin (sweet rice wine), 1 tsp sugar, 1 tbsp soy sauce, and 1 tbsp ketchup. Simmer briefly to create a cohesive sauce.
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15. Pour the simmered mushroom sauce over the hamburg steaks.
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16. Garnish with finely chopped green onions (to taste) and a sprinkle of coarsely ground black pepper (to taste). Serve.
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