Tear the bread into pieces and place in a small mixing bowl along with the milk. Set aside.
In a 12-inch straight sided saute pan over medium heat, melt 1 tablespoon of the butter. Add the onion and a pinch of salt and sweat until the onions are soft. Remove from the heat and set aside.
In the bowl of a stand mixer, combine the bread and milk mixture, ground chuck, pork, egg yolks, 1 teaspoon of kosher salt, black pepper, allspice, nutmeg, and onions. Beat on medium speed for 1 to 2 minutes.