Finely dice ½ onion, and mix 4 tbsp panko breadcrumbs and 3 tbsp dashi stock together in a small bowl to make a panade.
Place 200 g ground beef, 100 g ground pork and ½ tsp salt in a large bowl and knead them together until the mixture turns tacky and uniform.
Add the onion and panade to the bowl along with ¼ tsp ground black pepper, ½ tsp grated ginger root, and ½ tsp lard. Knead until evenly distributed.
Divide the mixture into portions and shape into ovals about 2cm (¾ inch) thick. Press a dent in the centers, then place them in sealable containers and refrigerate for 30 minutes.
Heat a skillet over medium and add ½ tsp cooking oil. Once hot, lay the patties inside with the dent side facing up. Fry until a light brown crust forms, then flip and repeat.
Pour 1 tbsp sake around the pan (not directly on the patties) and cover with a lid. Steam fry for 3 minutes, then turn off the heat and leave them covered for 2 minutes. Check that the thickest part reaches 75°C (167°F) on a thermometer.
Transfer the cooked patties to a wire rack to drain excess oil.
Wipe the excess oil out of the pan, but leave the browned bits and juices, they will add flavor to the sauce.
Deglaze the pan with 1 tbsp sake. Heat on low and gently scrape the bottom of the pan to release any fond. Add 2 tbsp dashi stock and 1 tsp mirin. Let it boil for 1 minute then turn off the heat. Add ½ tsp Japanese soy sauce (koikuchi shoyu) and 1 ½ tbsp ponzu sauce.
Gently squeeze the excess liquid out of 3 tbsp grated daikon radish (daikon oroshi) and prepare 2 shiso leaves. Plate up the hamburgs and top each one with a shiso leaf, a heap of grated daikon and some finely chopped green onions.
Serve with rice and your favorite sides. Pour the sauce over the top right before eating. Enjoy!
Notes
Salt the meat first, on its own: This order cannot move. The salt goes in alone, the meat turns tacky, and that tack holds the juices while the patties cook. Salt added later never gets it back.Cracking comes from under-kneading: Tossing the patty hand to hand matches the shape and the thickness. It pushes no air out. What holds a patty together was settled at the salt-kneading stage. Smooth cracks closed with a finger.When the mixture goes slack, add dry panko: The onion goes in raw, so it releases water while you mix. When it sags on your palm, work in unsoaked panko a pinch at a time.Do not push the heat high: Browning builds savor and tightens the surface. It seals nothing. Hold the pan on high heat and the juice gets driven out instead. Medium heat, and stop at a light crust.Grate the daikon right before you eat: A whole daikon carries no heat. The bite appears only when grating breaks the cells, and it is volatile, so it thins as it sits. Do not grate it the day before.Drain the grated daikon lightly: This sauce is thin to begin with, and undrained daikon leaves a puddle at the bottom of the plate. Most of the heat sits in the juice you squeeze, so wringing it hard costs you the heat as well.