Prepare the Panade and Onions: Soak the Panko breadcrumbs in the milk. In a skillet, sauté the finely minced onions until soft and translucent, then let them cool completely.

Mix the Patties: In a large bowl, combine the ground beef, ground pork, cooled onions, soaked Panko, egg, salt, and pepper. Knead aggressively with your hands until it becomes sticky, pale, and thoroughly emulsified. Divide into 4 oval patties. Toss them back and forth between your hands to expel trapped air.

Sear the Patties: Heat the vegetable oil in a large skillet over medium-high heat. Place the patties in the pan, press a shallow dimple into the center of each, and sear for 3 minutes until a dark crust forms. Flip them carefully.

Steam to Finish: Add 2 tablespoons of water to the skillet, immediately cover with a tight lid, and turn the heat down to low. Steam for 5 to 7 minutes until fully cooked and clear juices run when poked. Remove the patties to a plate.

Make the Demi-Glace: In the same skillet with the meat juices, pour in the red wine, ketchup, and Worcestershire sauce. Simmer over medium heat, scraping up browned bits, until the sauce reduces and thickens. Whisk in the butter for a glossy finish. Pour the thick sauce over the hot patties and serve.