Prepare the daikon oroshi: Grate the daikon radish using a grater, or use a food processor to finely grate it. Set aside.
Soak the panko: In a small bowl, combine the panko and milk. Let it soak for a few minutes until the panko absorbs the milk.
Cook the onion: Finely chop the onion and transfer it to a microwave-safe container. Add the butter and olive oil, then microwave for 1 minute and stir. Microwave for another 1 minute, or until the onion becomes soft and translucent. Let it cool slightly.
Make the hamburger mixture: In a large bowl, combine the ground beef, soaked panko mixture, cooked onion, sea salt, and black pepper. Mix well with your hands until the mixture becomes sticky and evenly combined.
Shape the patties: Lightly coat your hands with a little oil. Divide the mixture into 4–5 portions. Toss each portion gently between your hands a few times to remove air pockets, then shape them into oval patties.
Cook the hamburg steaks: Heat a frying pan over medium-low heat. Cook the patties for about 3 minutes on each side until nicely browned. Reduce the heat to low, cover with a lid, and cook for 6–7 minutes to steam and cook through. Insert a fork or skewer into the center of a patty. If clear juices run out, the hamburg steak is cooked. If the juices are cloudy, cook for another minute or two. Serve with daikon oroshi: Transfer the hamburg steaks to serving plates. Top with grated daikon oroshi, and drizzle with soy sauce or ponzu. Garnish with radish sprouts or shiso leaves if desired.