Measure 2 tablespoons of neutral oil. Heat half in a skillet over medium heat and sauté ½ minced yellow onion until soft. Let cool completely.
Mix 340g of mixed ground beef and pork with the onion, ⅓ cup of panko, 2 tablespoons of milk, 1 large egg, ½ teaspoon of kosher salt, ½ teaspoon of ground nutmeg, and black pepper. Knead until sticky and pale.
Shape into oval patties, tossing each between your hands to release air. Cover and refrigerate for at least 30 minutes.
Heat the remaining oil over medium heat. Brown the patties for about 3 minutes per side.
Measure 6 tablespoons of red wine. Add half, reduce the heat to low, cover and cook for 5-7 minutes, until the centers reach 160°F / 71°C. Transfer to plates.
Add the remaining wine, 1 tablespoon of unsalted butter, 3 tablespoons of ketchup, 3 tablespoons of tonkatsu sauce, and 3 tablespoons of water to the pan. Simmer until thickened and pour over the patties.
