1. Mince 1/2 onion finely (we call this cutting technique mijingiri in Japanese). Lay the cut onion flat side down on the cutting board. With the knife tip pointing toward the root end, make ⅛-inch vertical slices to within 1/2 inch of the root end. With the knife edge toward the root end, make ⅛-inch horizontal slices, again keeping the root intact.
2. Finally, make perpendicular cuts down through the vertical slices you made. If you need to chop the onions finer, run your knife through them using a rocking motion. Hold down the tip of the knife; otherwise, the onions will go flying around the room.
3. Heat a large pan over medium heat. When the pan is hot, add 1 Tbsp neutral oil. Then, add the onions.
4. Sauté the onions until tender and almost translucent. Transfer to a large bowl and let it cool.
5. Once the onions are cool, add ¾ lb ground beef and pork combination to the bowl. Add 1/2 tsp Diamond Crystal kosher salt, freshly ground black pepper as you like, and 1/2 tsp nutmeg. Tip: You can make hambagu with 100% beef, but it will not be as juicy and tender as combining the two meats.
6. Add the ⅓ cup panko (Japanese breadcrumbs), 2 Tbsp milk, and 1 large egg (50 g each w/o shell).
7. Start mixing it all together with a silicone spatula or spoon.
8. Then, switch to mixing by hand and knead the mixture until it‘s sticky and pale.
9. Divide the mixture into 4 portions (4 oz, 113 g each) or 6 smaller portions. Scoop out one portion.
10. Toss it from one hand to the other repeatedly about five times in order to release any air inside the mixture (see the video link above). Tip: Without releasing the air inside, the hamburger steaks will likely crack while cooking.
11. Make an oval-shaped patty and place on a tray or plate. Repeat with the remaining portions. Cover the patties with plastic wrap and keep them in the refrigerator for at least 30 minutes before cooking so that the meat combines well and the fat solidifies.